In fact, make changes in common to let the player understand the slider component.

In the UI. Internally it is still likert.

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.

It logically belongs there since it is no compound element and also 
needs to be near the other radio group.

This works similarly to the standard radio group, but has a fixed 
horizontal layout with images above the options.

The main components are right at top level and the directories below
mimic the data structure (unit->page->section).
The toolbar is only used in dev mode and does not belong directly to the
unit view.
The TextEditor is a complex component and deserves it's own directory
even though it is used within the unit-view. Might be refactored to its
own module later.

Elements can be dragged from the toolbox and will be placed where they
are dropped.

This is done by HTML drag and drop instead of Material dragNdrop. This
is because the setup is a lot easier and uncomplicated. In the future
the other Material drag and drop may be refactored away completely.

Also some small improvements to variable naming and file structure.

#23

- ... mostly for dynamic positioning
- Refactor reordering of sections. They can now be moved by small
buttons to the side, which appear when hovering. The section edit mode
is no longer available. -> KISS
- Removed all toolbars. The props can now be set via the props panel,
which now has sections for pages, sections and elements. This is to have
more vertical space available for editing the unit.
- Sections can now (only) be added via the toolbox panel, where the
second tab is now exclusively for sections. Future templates are
supposed to be whole sections, which can also be added from there.
- Refactor CSS layouting (flex and co) to have better handling of
vertically growing page (via sections)
- Some preparation for an upcoming overhaul of selection stuff
(again :))

Since this does not label anything but instead is meant for (longer)
texts.

Now there is a button at the side that can be used to slide the panels 
in and out.