This makes the list not collapse when used in dynamic context.

Those are wanted by the product owner.

This new setting makes child items space around in a row with line 
break.

Images in vertical alignment now always reserve the whole row and don't 
appear next to each other.

Generally only the editor needs to generate new IDs. Before there was
need to have it in common because some elements (like compound elements)
needed to generte new Elements including IDs.
Now all ID stuff is handled by the UnitService and the elements classes
are handed the fully built elements. Unfortunately this means that some
logic has to move back from classes into the UnitService. To un-bloat
the UnitService a (and in the future some more) helper class is created
for this logic.

The most important thing is, that the UIElement no longer generates
an ID when created. The ID has to be created before and passed to the
constructor.

- Now always starts with at least an empty list.
- Compatibility handling no longer sets invalid values. This was only 
temporary anyway but it is better to not do it at all.

The given serialized properties may not conform to all UIElement 
properties. They are merely meant as a building plan; they may also 
contain outdated properties which have to be transformed when reading.

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.