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.

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.

* Fix bug for small pages
* Add functionality for tabbed pages
* Use layoutContainer as container for intersection detection
* Rename properties and methods

Also some small improvements to variable naming and file structure.

#23

Move `MatDialogModule` from editor to common
Add `AlertDialog` to player

- 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

Merge the `unitDefinition` with the `unitState.dataParts` to insert the
saved values

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.

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.

* Remove workaround with `Promise.resolve().then(() => ...`
* Use `changeDetectorRef.detectChanges()` after updating the validators

* Since always visible pages are removed from the order, form
arrays must be rearranged
* Refactor getter as properties in `PlayerStateComponent`. Getter
work like functions and are executed every time Angular change
detection runs.

* Create `FormGroups` without ids
* Add `FormArray` for elements

* Add section array to the form groups of the pages
* Remove unnecessary component factory from `SectionComponent`

Replace pageIndication with translate pipe

Refactor some translations

more information about [ngx-translate](https://github.com/ngx-translate/core)

* Move setting of validators from `FormElementComponent` to
`ValidationMessageComponent`
* Add validators and warnings to `ValidationMessageComponent`
* Move subscriptions for form controls and their validators from
`PageComponent` to `FormComponent`