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.

to prevent vertical scrollbars when using default values.
* Increase height for TextFieldElement, TextElement and DropdownElement
* Change HTML structure of TextElement

This enables to hightlight parts of the text with colors.

This uses kolkov/angular-editor.

This is in preparation for dynamic layouting.
The actual dimensions are set by the overlay component which differs but 
is existent for both editor and player.

Also refactor a lot of stuff belonging to this:
- You can edit the default values via properties panel
- All elements now again have a common parent directive. form elements
have a special parent for forms.

* Only elements which has to be registered by the player's form inherit
from `FormElementComponent`
* Element components use their specific `elementModel`
* Rename file for `FormElementComponent`

Since this does not label anything but instead is meant for (longer)
texts.

The UIComponents now only have the properties they can make use of. This 
means that the angular components use the CSS properties explicitly 
instead of having all kinds of useless CSS properties.
Also it is no longer needed to update the CSS styles via event because 
Angular's change detection takes care of that.

This makes the actual component elements agnostic to any drag and drop 
and selection logic, which is handled completely via the 
wrapper-overlay. Now the component elements can be easily used by the 
player.