- Move props to separate files
- Fix typing

Also improve media element property control layout.

#204

Casting to ButtonElement caused issues where changed values were not 
recognized by inputs.

#280

- Missing type inforamtion as constructor parameter
- Also the new ID generation mechanism uses the type of the element to 
request an ID from the the IDManager. Here was a mismatch, where the 
type is 'likert-row', but the IDManager only knew 'likert_row'. This is 
now files all IDs use hyphens instead of underscores.

Remove 'any's be adding proper types and using 'never'.

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

- Elements initialize their fields explicitly and not with
  Object.assign, since Object.assign also assigns
  (and therefore overwrites) parent fields.
- IDService is no longer an Angular service but a basic singleton. This
  allows to use it in classes.
- The IDManager can be optionally passed to unit and element
  constructors. When present the elements check and fix their given ID
  when created.
  This way elements can be created without depending on the external
  IDManager but also with the dependency.
- ElementFactory is no longer used to create elements. Those are now
  either created directly via 'new' or via the Section, which knows the
  different element types.

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.

- Add fieldset and reorder fields
- Add dynamic connectedDropLists menu

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