Before this commit, untagging 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 removed.

This commit allows for untagging multiple resources at once,
speeding it up a lot (the bigger the database/resources_tags
table is, the more speed increase you get)
and makes sure the signals sent by the models are correct
and match the removed rows.