Bug 722975 - crash when removing tag from palette while filtering by same tag

Each of the following cleans up tag refcounting, fixes access to
released memory, or other small glitches. Not sure which change
actually fixed the bug:

gimp_data_remove_tag(): remove the found tag, not the passed in tag
(which is to be treated only as a value for comparison).

gimp_tagged_remove_tag(): don't continue the loop after the tag to
remove has been found, there can only be one matching tag, and the
list element has become invalid.

gimptagentry.c: keep references around for the members of
entry->common_tags, and make sure the references are always dropped
properly. In assign_tags(), reference the "add" and "remove" lists for
paranoia and safety reasons.