This enables to hightlight parts of the text with colors.

When this is set the page will only be rendered to that width. Otherwise 
100% of the available space is used.

This property should be used when an always visible page is used and 
states the ration of the always visible page to the other pages.

Before it was the index of the option. HTML UI Elements take the actual 
value as parameter anyway so no need to use any index.

The section now always has the same width as the page (minus margin).

Can only be true/false.

Also some small improvements to variable naming and file structure.

#23

- Add (editable) rows to the grid
- Add drag and drop functionality to grid elements. The element is in 
'absolute' positioning mode while being dragged and snaps back to the 
grid when droppped.

Property to define the position of always visible page

Sections and elements can have this prop. Elements only need it to 
determine what properties to show in the props panel.
The others are basic props for grid layouting. Refer to the grid 
documentation.

Replace pageIndication with translate pipe

Some elements were missing parent interfaces.
Also refactor unit factory to not return objects completely adhering to 
interfaces but records, which then can be put together to create the 
real object adhering to the interface.

Those props are set when page is created and can not be changed. This 
will have to be refactored when the functionality to switch pages is 
implemented.

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

The exact type of the value is still specified in the child class.

Those have a required setting and warn message.
- Also refactor SurfaceElement to no longer inherit from UIElement as 
this is not needed since we can have multiple inheritance for 
interfaces.

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

One page can have this setting and is supposed to be shown at all times 
on the left side.

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

* The wrapper of the player component listens to the `valueChanged`
event and posts a message to the host of the aspect-player
* Improve code style and remove lint errors

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 defines the elements priority when stacking elements on the canvas. 
Directly maps to the CSS property.

Also show tooltip on the input to explain what it is for.

This is set implicitly by the sum of section heights.

There it can be used by the player as well.