#10 Allow _adding_ tags to a group of photos

[Jens Müller]

When I mark several photos which already have tags (may
they be different or equal, doesn't matter) and clik
"Edit", I can set the tags, but they will overwrite the
existing ones.

It would be nice if one could add tags.

Maybe tags which all marked photos have could be black,
and tags which only some have could be gray. If you
don't modify a grey one, everything remains as it is,
if you add that tag, it get's black and the tag is
added to all marked photos, and if you remove the grey
tag, it get's deleted.

A few words to the use-cases: I think it's quite common
to mark many photos with the name of a town and then
some of them with the name of a building or something.