- Basic implementation

Also restructure files containing interfaces.

- Add ReferenceManager to check if deleted elements are referenced 
anywhere in the unit. If so display a warning.
- Move page logic to UnitService

- Background color must not be transparent for the stickied panel to 
overlay properly.

- Use TextImagePanel where possible

Less unneeded scrollbars and fixed height.

- Remove global dialog style

- Reformat styles

- Reformat styles

- Reformat styles

- Reformat styles

- Reformat styles

- allowReplacement (moved from element to list prop)
- originList and Index are non-optional (means placeholders have to be 
used sometimes)
- Re-introduced the dict of all DropListComponents. This is the only way 
I have found to get a reference to a DropListComponent by ID. This is 
needed for returning to origin. Origin is not actually part of the 
transaction and needs to be fetched from somewhere else. Hence the dict.

- Console statements still present for debugging.

Allows DropListvalueObjects which have this setting to be replaced in
lists which only allow onlyOneItem. When this switch is set, the class
sets the other 2 internal values
(originListID?: string; originListIndex) itself. Those can not be seen
nor changed from outside.

Also refactoring in lots of places around droplists. Better names, 
cleaner structure, comments.

#305

Reads file as base64.

Also rework the GeoGebra dialog to close automatically when a valid 
definition is available, either through pasting or file upload.

- A HotspotImage has an image and divs placed on it, which are used
as hotspots
- Inherits from FormElementComponent, which is why the hotspots are kept
  as a list in the value property.
- Can be validated as required and uses its own custom validator

Options are now possibly rich text. This needs to be taken care of.
Also use extra span tag to preserve all whitesoaces in the index-suffix.

#279

- remove ID handling from model classes. This is now done by the editor 
itself (mainly unit service).
- Rename and move IDManager to editor as IDService

Also add it to the likert row dialog.

#112

Now properly finds all normal and simple drop lists.
This is achieved by a new method of unit all it's sub-components, which 
returns all contained elements with and optional type filter.

Also improve (still not very pretty) styling of the drop list props 
panel.

#260 #259

Add 2 new menu buttons to the section menu.
One simply copies the selected section to the clipboard.
The other opens a dialog where a copied section can be pasted. The 
section element's IDs are checked if they are available and a warning is 
shown.

This way elements can handle their logic themselves without having to
rely on outside utility classes.

Also restructure files in common in a more logical way.

since the player uses it as well