The DOM query is now only done once and nt again everytime the value 
changes.

* Rename service to `KeyboardService`
* Replace the two fixed rows for upper and lower case letters
of the keyboard with a general row array
* Rename css classes, variables and methods

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

Overridden by some child elements (TextField, CheckBox)

* After deleting validation component, element component is no
longer needed and is deleted
* Element overlay takes over the methods of element component

* 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

Accidental closing of the keyboard can happen when the user clicks on
the board under the keys

* Bugfix for units without always visible page
* Bugfix for aspect ratio over 50%
* Integration of keyboard

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.

Also add release notes document.

The duplication is done by creating a new element (UnitUIElement)
for each element of the copied object. And then overwriting it with
the props of the old one, in addition to assigning a new ID.

Can now be repositioned and moved between sections properly.

- The selection service only needs to handle element selection. For
pages and sections having the index is enough.
- Those indeces are simple fields instead of observables. This also
simplifies the components which don't have to get the value out of the
observables.
- Therefore the selection service does no longer need to have a direct
reference to the unit service and it's unit.
- Selection logic is not done by the section components themselves but
by the parent canvas component.
- The unit service can now manipulate pages and section by index in
addition to passing the necessary objects directly.

* Fix scrollbar bug
* Remove using of timeout method
* Add Material fonts

Because the always visible page is now positioned at the first place of
the unit definition.

* 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

Commenting the type check out for now. I suspect we might need it again.

When maxWidth is not set it now correctly ignores the prop is takes the 
default 900px.