flex-start is the correct value for flex containers.

Measure holds the value and a unit for CSS properties.

- Reformat styles

- Reformat styles

The native browser-drag-and-drop does not work in tablets. Therefore 
back to Material which works.
Features are moved over with the exception of replacing items.

- Copy-Lists now always allow putting items back.

- Renew example files
- Update versions and notes

Uses external software 'MathLive'.

Distpacker issues with mathlive have been solved in a custom 
distpacker.ts in scripts-folder. This is now used instead of the 
distpacker from the npm package.

- Remove unused variable
- Add typing

- Part of the properties panel for every element (like delete and 
duplicate)
- This allows interacting with elements, generating events and inpect 
their usage. Usually this is unwanted since it might interfere with 
dragging the overlay, but sometimes you want to get a feel for the 
element or check it's contents (GeoGebra for example)

- Replace Material Droplist with native HTML events
- Remove simple-drop-list element; Cloze elements now use the normal 
DropList element
- Add example units

- 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

..instead of multiple methods doing almost the same stuff.

#264

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.

Components with bigger scope are put to the component root, canvas, 
props panle and new-element-panel.