- 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.

This is the same hack used in the editor for overlay elements. Here 
applied to the actual dnd elements seen in the player.

Explanation: The cursor can not be applied to the dragged elements as it 
is not positioned (at least that is my understanding). The workaround is 
to style the body (cursor) while anything is being dragged.

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.

This allows to use them in template bindings. Used in compound elements.

- List items only have a margin between then but no longer on the 
outside.
- Remove unused background color. This is set dynamically in the 
template.

This shows a outline on potential dropLists when dragging an item.

Options and value were the same thing anyway and the value was updated 
once anything was changed within that dropList.
The player though needs a start value and this way we have one.

- Add background to dragged element via cdkDragPreview
- Add placeholder box for dropping via cdkDragPlaceholder
- Add animations for transitions when dragging and dropping

The drop listener simply does nothing when this is set. This way it is 
still possbiel to drag item but dropping them has no effect.

Also improve conditional border style. The additional dragPreview is 
needed so the dragged item does not have the border.

The width/height of 90% are a preliminary workaround to not produce 
overflow with the border. A better solution would be good.

This element has a list of string as editable options. Those strings 
form draggable elements which may be reordered or moved between 
different DropList elements.

Also refactor option change logic to allow any property not just the 
text property.