- Remove element as parameter

- Implement marking buttons as toggle buttons

- Check if the selection refers to the content of the text component

This disables the possibility of nested markers

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 controls the general font size. Specific spans can still be styled 
individually in the text editor.
- The TextEditor also uses this font size as default when rendering the 
text.

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.

Highlighted (background colored) text is done with mark elements which
don't have a color and make the contained text lose the styling of the
parent span, which has the text-color property.
To solve it we simply inherit the color of the parent span.

Setting on type button avoids that click events are evaluated from
the surrounding form

This allows to use them in template bindings. Used in compound elements.

The MarkingService now takes care of the selection and its validation.
The text component sends its change via elementValueChanged.

This function will be implemented differently.

* Refactor markable text
* Prevent multiple selections (feature is only supported in Firefox)

And avoid global styling.

This way empty paragraphs still produce an empty line.
Not possible to do inline because CSS pseudo classes do not work inline.

This improves bold text readability.
This has to be done in the actual element, in the TextEditor and on the 
properties panel.

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.

Add styles for bar and buttons

- Remove interfaces
- Add classes for all elements as well as unit, page and section
- 2 intermediate interfaces are kept. They dont constituate a viable 
object but only additional properties, like background-color. Those can 
be added as interface to all elements. Initialisation has to be done by 
the element itself though and not by some constructor higher up.

to prevent unnecessary scroll bars from appearing

This is preliminary and will be reworked in the future.