Add the ugliest 'delete dive' model ever

This interface works the same way the "edit dive" menu item does: it's a
text entry meny item on the dive text entries (ie buddy/divemaster/notes
sections).  Except you pick the "Delete" entry rather than the "Edit"
entry.

It kind of works, but it really is a pretty horrible interface.  I'll
need to add a top-level dive menu entry for just deleting all selected
dives instead.  And it would be good to be able to get a drop-down menu
from the divelist instead of having to do it from the dive text entries,
which is just insane.

But that requires gtk work.  I'm not quite ready to get back into that.
Thus the "exact same insane interface as the explicit 'Edit' mode".

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>