don't change due / nextwork date when nextwork date > due date, just
set a property to allow highlighting

it's impossible to know which one's wrong, so just point out they're
wrong and let the user fix it

Leo build: 20141008225053
works with new code in todo.py (previous commit)

not sure if it works with qtcreator though

Leo build: 20141008225444
Merge branch 'master' of github.com:leo-editor/leo-editor

Conflicts:
	leo/core/commit_timestamp.json

Leo build: 20141008230223