Before this commit, tagging resources in
KisTagResourceModel would happen only one resource
and one tag at a time. This was pretty inefficient.
Moreover there was an issue with signals sent by the model,
they didn't match the actual rows being inserted or removed.

This commit allows for tagging multiple resources at once,
speeding it up a lot (the bigger the database/resources_tags
table is, the more speed increase you get; with Rakurri's
bundle imported, I would get even 140x speed increase)
and makes sure the signals sent by the models are correct
and match the inserted and removed rows.