Improve MainTab behavior when using the scroll wheel

This gets the behavior close to what we really want.
- scroll wheel no longer enters edit mode when over the tabWidget
- scroll wheel doesn't modify dateTimeEdit, nor does it enter edit mode
- scroll wheel still scrolls both the notes and the full widget

The only oddity is that when clicking on either the dateTimeEdit or the
tabWidget we don't immediately turn on the 'being edited' warning (as we
do for all the other widgets). For those two widgets the user has to press
a key before edit mode starts.

I think this Fixes #176

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Ideas-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>