The updateListener does not detect all changes inside a GeoGebra element

#309

The API has been updated and some commands and fields are removed. They 
are kept in for compatibility.

Re-implement DropList element

Reads file as base64.

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

- 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)

Usually all errors should be caught by the sanitizer, but if that fails 
the editor will be in an okay state.

Also set geometry element to always use fixed size, which is all that 
works anyway.

...but disable the list auto-generation for bullet and ordered lists.

#226
#369

#370

This way the value of the selection box is displayed initially

- Remove property dropListIDs
- Remove mousedown event

Otherwise, no elements can be dropped at this place

To detect if a custom form element was touched it is necessary to set a
custom tab index

... to create new references for the connectedTo array.

This version is unreleased and can be used.

Use the source list placeholderIndex instead of the index for the target 
list.

Chromium does not allow DOM manipulation on dragstart. Therefore add 
timeout.

Duplicate value objects without keeping the reference to the old object.

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

- Rewrite interface 'VopMetaData'
- Add parameter 'metadata' to VopReadyNotification

- TextAreaElement
- TextFieldElement
- TextFiledSimple
- SpellCorrectElement

- Replace type 'any' with 'unknown'

- This corresponds to the representation in the keypad
- Prepares further adjustments to the keypad