Correct the start time of planned dives

Subsurface follows the lead of most divecomputers to use times without
timezone - so all times are implicitly assumed to be local time of the
dive location; so in order to give the current time in that way we
actually need to add the timezone offset.

Instead of relying on OS specific members of struct tm we use the glib
timezone functions to get the timezone offset for us. Of course, the
function used to do this is only in glib 2.26 or newer, which once again
means that Debian stable won't be supported. But since that doesn't build
other parts of Subsurface, either, I think I'll live with that.

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