Shift times of selected dives

This patch adds the possibility to shift the times of all selected dives
by a fixed amount to correct for time zone problems or mis-set dive
computer clocks.

Select the dives and right click in the dive list.

[Dirk Hohndel: added .ui file to FORMS and fixed some whitespace damage]

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Improve shift time functionality

In commit 85fab31c71fc ("Shift times of selected dives") we added a dialog
that didn't align its content very well. This change improves the layout
(I hope) and makes the labels in the layout clearer (earlier and later seem
easier to correlate with the direction of the shift).

The original commit also forgets to deal with the consequences of shifting
the dives. The dive list needs to be re-sorted (as the relative order of
dives could have changed) and be marked as changed. And we should try to
maintain the selection across these operations.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>