* Adapts their appearance to mat-form-fields
* Fix coloring of background for checkboxes

Overridden by some child elements (TextField, CheckBox)

* Delete validation message component
* Use instead `mat-error` inside element components to take advantage
 of `mat-input-field`
* Add get validators method to each form element component
* Display validation messages via `error-transform` pipe

Since the material elements don't give a way of applying the background 
color, a hack is needed to get the internal elements and apply the 
style.

Directive to be able to do it for text field and text area.

* Remove unnecessary styles
* Refactor case distinctions
* Improve styles of some elements

Refactor styles for elements and element overlays to achieve stretching
of dynamic sections when text areas are resized

to prevent unnecessary scroll bars from appearing

This is preliminary and will be reworked in the future.

Consider the available space for width and height
Increase their default heights

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.

The urls must be declared as trusted to be accepted by the application.

Also some small improvements to variable naming and file structure.

#23

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.

Both use the property name 'min' and 'max' but map it to min/max and
minlength/maxlength respectively.

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.

* When creating components for each element a
`ValidationMessageComponent` is also generated dynamically
* Prepare usage of default values for form elements
* Add `form.ts` for form specific interfaces
* Rename some variables and properties

Use `elementModel.label` instead of `elementModel.text`

* 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.

Use `parentForm` input in `PageComponent` and in `SectionComponent`
instead