Autofill GPS coordinates for added dives.

Prior to commit 95cb4e, when a new dive was added with the same location
name as a previous dive, the GPS coordinates for that new dive would be
automatically set to that of the matching previous dive.

This restores this feature, by duplicating code further down
qt-ui/maintab.cpp that handles the case where multiple dives are
modified at once.

Signed-off-by: Gaetan Bisson <bisson@archlinux.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>