This controls the general font size. Specific spans can still be styled 
individually in the text editor.
- The TextEditor also uses this font size as default when rendering the 
text.

Use a copy of the original elements to avoid accidentally changing them.

The change of moving some properties to sub objects broke the creation 
of the combinedProperties object, which can't deal with nested objects. 
To circumvent that all elements are now flattened before being combined.
The connected property panels now again look for the properties on the 
element itself and not in sub-objects.

If the property is set, the video will be scaled as far as its
container (static element or grid) allows, otherwise only up to its
maximum native size.

If the property is set, the image will be scaled as far as its
container (static element or grid) allows, otherwise only up to its
maximum native size.

- Dnd values now have a special type that has an ID, a string and
  another string for an image path.
- A new dialog exists for editing thoe options. Similarly to the likert
  header.

- Fix the name to not have redundant 'component' suffix
- Move template to its own file

Broke during the refactoring of the element models. Now correctly uses 
the positionProps field.

Also improve variable naming.

registerLocaleData is needed to set the thousands' separator. It has called once per application and should not stay in SliderComponent (although no other component need this by now).

MatButtonModule is used in all projects!

The interaction can now be turned off without hiding the button.

The problem being solved is that compound-sub-elements must not have
all the properties other elements have. For exampe positioning info.
Sub-Elements are positioned inline. To avoid having similar class
inheritance structures for positioned and inline elements, we use
interface. Positioning is done via interface and is therefore optional.

Those optional properties are kept in an object in a variable. This
allows to hide specifics from element classes and removes the need to
initialize all the fields in every class using the interface.
The elements mostly care about their own specific properties anyway.

The same technique is used for font and surface elements. Here we also
don't want to initialize all the styles in every class.

Changing values of element properties works like before. Just set the
property as if it would be a direkt prop of the element. The element
itself puts the value in the currect sub-object (positioningProps,
fontProps etc).

For reading the values there is a similar method, but it is not used
anywhere by now. Since the properties panel operates on the raw values
anyway (refer CombinedProperties) and for element components this would
mean calling a function in the template, which caused change detection
to run the function very often and is therefore bad practice.

Additional notes and refactorings:
- Restructure all files in common. UIElements in the same folder etc.
- Move all interfaces to the UIElement file. This avoids circular
  imports, which would be needed to allow PositionedElement to extend
  UIElement.
- Interface initializers also handle reading values in the old form.
- Add PositionedElement for use in canvas overlays. This is a UIElement
  with guaranteed postionProps.
- Don't export all Material packages from common, only the ones
  used in player and editor. The rest is only used in common and
  does not need to be exported.

Highlighted (background colored) text is done with mark elements which
don't have a color and make the contained text lose the styling of the
parent span, which has the text-color property.
To solve it we simply inherit the color of the parent span.

When using borders deselecting would remove the borders of the 
(text)element as well.

The tooltip declarations does not need escaped text. Was shawing one to 
many backslashes.

Min-height should be optional for dynamically positioned elements, as 
you usually don't need it and it causes more problems than it solves. 
Elements with min-height have cause unexpected margins between elements.

The font size is applied by setting the list style. This is a 
workaround, since the list extension does not support setting font size 
directly. It takes the selected font size (which is now saved ina 
variable) and applies it. This is indepent of the font size of the text 
in the list. The item font size is done individually with a span anyway.

Also font size and list style droplist now (re-)apply the value on click 
of an item. This allows updating font sizes and list styles without 
having to select another option first.

- Add a new variable to save the selected font size and reuse it for 
list styles.

(when not set)