First step towards grabbing keys and handling them ourselves

This commit steals the cursor up and down keys away from gtk so regardless
where gtk thinks the focus may be, we can still use the keys to change
between dives.

In the current UI design where all editing happens in separate windows
this works as expected, as we only grab the keys for the main window. If
we manage to re-enable in-place editing then we need to make sure that
this doesn't cause problems (as gtk uses up/down for the ability to change
drop down selections in combo boxes or values in spin buttons. So we must
make sure that we stop stealing these keys once we start editing something
(in which case simply switching to the next/prev dive wouldn't be a good
thing, anyway).

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Use the Left and Right keys to switch between divecomputers

The existing code had the somewhat retarded Ctrl-C binding for displaying
the next divecomputer and no way to go back to the previous one. With this
commit we use our keyboard grab to map Left and Right to previous and next
divecomputer. Much nicer.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Merge branch 'updown'

Bring in the keyboard handling change