dom-text-mapper:
 - New feature:	provide	API to notify all dom-text-mapper instances about a change in the DOM.
 - New feature:	provide	API to get mappings for	a given	DOM node. (Until now, we only had this for xpath.)
 - New feature:	provide	API for	reading	the current content of the selection, with the usual conversions. (Used	to d\
o this internally, but now the feature is exposed.)
 - Optimization: in performUpdateOnNode, when reading node content, tell it not to bother with restoring selection a\
fterwards
 - Bugfix: in performUpdateOnNode, don't try to	update data cache if it	does not exist yet
 - Bugfix: in performUpdateOnNode, if we are passed a node that	has not	existed	previously, check it's parent, inste\
ad of dying
 - Error handling: detect wrong usage of saveSelection / RestoreSelection
 - Added some missing comments