This looked bad having a null value in the DOM.
16 is the default font size of the app right now.

- Reordered the buttons to make it look nicer (but still make sence 
logically)
- Put some more divs with flex layouting to prevent elements moving 
unexpextedly.
- Changed color and list inputs to split-buttons. Those compone a button 
with a dropdown.
- The editor now saves the set color and  it can be applied to 
subsequent selections as well.

ProseMirror seems not to allow editing text nodes. Since a text node can 
be a child of an element being applied the bullet list to, it fails for 
that text element. It seems it was workingly correctly before. The error 
seen on the console was inconsequential since we don't want to apply 
attributes to pure text nodes anyway. This change at least removes this 
error.

The icon does not make it sufficiently clear what the purpose is.

And avoid global styling.

This improves bold text readability.
This has to be done in the actual element, in the TextEditor and on the 
properties panel.

Since no default size is set, the default font size is set to 16px, 
which is the browser(?) default on my PC.

Before it was done with a dysfunctional hack. Now with proper extensions 
to existing (tiptap-node-)extensions.
Also reads existing style and sets select-element accordingly.

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.