Changing Zero Width Space handling strategy

ZWS allows to put caret into empty inline node in Chrome

This change goes from:

- content observer puts ZWS on nodes set to ''
- ZWS stays in nodes and is handled accordingly (selection, arrow keys,
backspace/delete keys etc.)

to:

- content observer puts ZWS on nodes set to ''
- content observer removes ZWS from nodes which .data changes
from 'ZWS' to other string
- special treatment of ZWS is necessary only on "empty" document text
elements, ie those whose container .data equals ZWS