Add SessionBackend abstraction

hides away any assumptions about server class structures from the Editor class
Remove option to configure the member id in local edit mode
Merge pull request #624 from kossebau/sessionbackend

Reduce assumptions about backend in the editor
Ensure a caret doesn't receive any new events after destroy is invoked

Deleting the caret before calling ensures there is no way for any external
sources to get access to the soon-to-be destroyed caret.

As part of tracking this down, scheduled tasks will now throw an error
if an attempt is made to trigger the task (either delayed or immediate)
after the particular instance has been destroyed.

This then showed that functions bound to browser events swallow errors
quietly, so additional logging was added to the EventManager delegation
code to ensure errors generated when processing an event are logged.
Merge pull request #620 from peitschie/caret-destroy-fixup

Ensure a caret doesn't receive any new events after destroy is invoked
Split Editor into Wodo.Texteditor and Wodo.CollabTexteditor
move fullscreen pinch-zoom into own class FullWindowZoomHelper
add product wodotexteditor
Add HOWTO-wodotexteditor.md
Add README.md to programs/editor