Improve detection of modified drafts

If the target tiddler of the draft is missing (as happens after
clicking “new tiddler”), then we count the draft as modified if the
text is not empty.

Also we count the the draft as modified if the draft title has been
changed
More resilience to Tiddler.isEqual()
Smarter cancel edit behaviour

If we cancel editing a tiddler that doesn’t exist, then we no longer
show the resulting missing tiddler.