The material dialog functionality used, adding the mat-dialog-title 
directive the toolbar.

The text area was mistakenly shown.

There were several issues with it's properties, which could not be 
changed.
Now it is properly initialized and prop change get actually handled.

This helps identifying the selected element.

For some reason the 'host' selector does not work properly. Switching 
back to using the component selector.

This can be enabled for TextFields and TextAreas and shows a keyboard
at the bottom of the screen. This does NOT set the field to read-only
which is probably wanted in a situation where the system keyboard is not
to be used. For flexibility purposes it is not implicitly set and needs
to be set manually. The same goes for other input assistance settings,
which should not be used in addition to the keyboard but it is allowed.

The keyboard has an extra switch to additionally show french special
characters.

Don't use the page maxWidth when it is not set.

This makes newly created elements appear next to each other instead of 
on top.

It is enough to set the row and column values. Spans are now only 
changed from the props panel.

This had to many implementation problems and is not really an important 
feature. Removing the corresponding stylings also improves the visual 
layout - elements can now grow freely and the section resized with them.

This is now done only via the cdkDropListGroup directive which collects 
child drop lists. There is actually no need to connect them manually.

This makes the value unselectable.

Fixes: #203

gridColumnStart: number;
gridColumnEnd: number;
gridRowStart: number;
gridRowEnd: number;

becomes

gridColumn: number;
gridColumnRange: number;
gridRow: number;
gridRowRange: number;

Improve code and prohibit resizing of section when child element is 
dragged.

Now uses the implicit grid in auto mode.

Change translation key and meaning from 'typeAreaWidth' to
'sectionWidth'

Remove case distinction when labeling the page's margin width

Instead, use Textfied component with the property isClozeChild

Now named just position.

The unit definition now has 2 meta-fields: 'type' and 'version'.
The former has the fixed value of 'aspect-unit-definition'.
The latters contains the version without any prefixes, i.e. '1.2.3'.

The sanatizer tries to read the versions of older formats as well and
uses the found version. This is important since the handlePlusOne
method must only run of a specific version (in the old format).

The sanatizer no longer shows any messages and therefore does not need
the MessageService anymore. The information about whether sanatization
happened or not is returned as second parameter for those that are
interested. The editor is, the player is not.

The editor API service now constructs the 'unitDefinitionType'
parameter from both values, resulting in something like
'aspect-unit-definition@1.2.3'.

Use the actual unit reference from UnitService instead of a possibly not 
up to date local variable.

The width of a page results from the value for the maximum width plus
twice the value for the spacing

- Units and element are now built via the factory. This ensures only the
  wanted properties are present. All others are discarded.
- Since the sanatizer is supposed to check the unit defintion version
  it is used by player and editor and therefore put in common.
  This in turn makes it necessary to put the JSON resolver settings
  in the root-TSConfig file.
- Renamed a couple of properties to have clearer names.