- Use fixedSize always together with dynamicPositioning!
- Rename css class 'center-content' to 'fixed-size-element'
'fixed-size-element' ensures that the content does not overflow.

Remove the wrapper divs in each element component and add the
'fixed-size-content-wrapper' class to the element overlays
of player and editor

The previous solution, which uses a top position of 50%, did not work on
iPad

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.

- Simplified the code logic. Verification is done once in the beginning 
and not on every element.
- Now uses semver to verify and compare version strings

ImportModuleVersion stores the unitDefinitionType of the loaded unit
definition. During the initialization of the unit, the individual
elements can react to the stored value.

This is not a complete backwards compatibility. Only the value "0" can
be safely classified as false. It is therefore set to "1".

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.

This reverts commit 87534c53.

Options still had a margin-top, which is wrong when there is no label 
above.

The theme file is now project specific because Angular demands assets
to be part of the project.
It also differs in content: The editor theme is only applied to canvas
elements. For the player every checkbox and radio button is affected.

The components now again use the default color of 'accent'. The custom
theme now has the green as accent color and no longer primary.

Used in checkboxes and radio buttons, which are now green.
The secondary is unused but for safety set to the same as the default
theme should be.

For all elements using radio buttons.

Also remove duplicate CSS declaration.

The given serialized properties may not conform to all UIElement 
properties. They are merely meant as a building plan; they may also 
contain outdated properties which have to be transformed when reading.

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.