This is done according to the Angular best practices and minimizes
namespace collisions when using other packages which might use the same
selectors.

Technically the selectors are not even becessary since we use an Angular
component factory to create element components but it is still useful
for the DOM element naming, for debugging purposes for example.

Compound elements are now supposed to use overlays for their child
elements. This overlay makes selecting child components (by clicking on
them) and marking them as selected (done by the selection service)
possible.
The SelectionService no longer needs special logic to handle compound
children selection, as they now also have an overlay with the same
interface as normal (canvas) elements.

A few modifications in connected directives are necessary. The now
handle children components in a proper array instead of QueryList.

Likert elements do not have clickable children yet and work a little
differently. This should probably be unitized in the future.

Elements are supposed to be centred when this setting is used. Since it 
is the same CSS for almost all element components it is put into global 
styles.

Missing components: video, spelling.

The problem being solved is that compound-sub-elements must not have
all the properties other elements have. For exampe positioning info.
Sub-Elements are positioned inline. To avoid having similar class
inheritance structures for positioned and inline elements, we use
interface. Positioning is done via interface and is therefore optional.

Those optional properties are kept in an object in a variable. This
allows to hide specifics from element classes and removes the need to
initialize all the fields in every class using the interface.
The elements mostly care about their own specific properties anyway.

The same technique is used for font and surface elements. Here we also
don't want to initialize all the styles in every class.

Changing values of element properties works like before. Just set the
property as if it would be a direkt prop of the element. The element
itself puts the value in the currect sub-object (positioningProps,
fontProps etc).

For reading the values there is a similar method, but it is not used
anywhere by now. Since the properties panel operates on the raw values
anyway (refer CombinedProperties) and for element components this would
mean calling a function in the template, which caused change detection
to run the function very often and is therefore bad practice.

Additional notes and refactorings:
- Restructure all files in common. UIElements in the same folder etc.
- Move all interfaces to the UIElement file. This avoids circular
  imports, which would be needed to allow PositionedElement to extend
  UIElement.
- Interface initializers also handle reading values in the old form.
- Add PositionedElement for use in canvas overlays. This is a UIElement
  with guaranteed postionProps.
- Don't export all Material packages from common, only the ones
  used in player and editor. The rest is only used in common and
  does not need to be exported.

No unused space around images and image an text below each each other.

This reverts commit b5d3b810.

The old behaviour with non-scaling images is wanted here. Will improve in subsequent commit.

Also all elements now take the parentForm as input. Allowing it to use 
them with template binding and not only setting the value directly (in 
the controller).

- Images now scale up and down depending on the available space.
- The new property controls the first (column) size parameter for the 
grid. The others are fixed at 1. Decimal values are allowed for 
fine-tuned control.
This prop is passed on to the child likert-row component. It uses the 
same grid and needs this info for proper alignment with the column 
headers.
- Remove unnecessary div in likert

All elements now use the elementValueChanged event. Also
formValueChanged is changed.

Use flex to position image and text below one another. Don't use any 
image scaling setting. Now the images grow but don't shrink.

There were some things missing when renaming questions/answers to 
rows/columns. Now everything is properly named.

Since this is built using a grid it does not need dimensions. In fact it 
having dimensions interferes with styling.

* Add IntersectionService
* Add CompoundElementComponent as parent for LikertComponent
* Inject ElementRef to common ElementComponent
* Rename players' ElementComponent to ElementContainerComponent

Other elements do implement the interface but do nothing with the 
property. This make all the interface handling a lot easier at the cost 
of having useless variables in some contexts.

* Remove FormService Injection
* Move FormService and form types to player
* Move interface ValueChangeElement to uI-element.ts
* Use interface InputElementValue instead of
string | number | boolean | null

* Add `InputElementValue` as type when needed

Colors and font stuff.