Desktop: Fix Gas Editing for Manually Added Dives.

- show the correct gasmix in the profile;
- make gases available for gas switches in the profile after they have
  been added;
- persist gas changes;
- add air as a default gas when adding a dive.

This still has problems when undoing a gas switch - instead of
completely removing the gas switch it is just moved to the next point in the
profile.

Signed-off-by: Michael Keller <github@ike.ch>

desktop-widgets/mainwindow.cpp

@@ -711,6 +711,7 @@ void MainWindow::on_actionAddDive_triggered()
 	d.dc.meandepth.mm = M_OR_FT(13, 39); // this creates a resonable looking safety stop
 	make_manually_added_dive_dc(&d.dc);
 	fake_dc(&d.dc);
+	add_default_cylinder(&d);
 	fixup_dive(&d);
 
 	Command::addDive(&d, divelog.autogroup, true);