Partial merge of code from Aurimas Juška.

* app/core/gimptag.c: A new minimal GimpTag type with only a
gimp_tag_equals() class function so that we can

* app/core/gimptagged.c
* app/core/gimpdata.c (gimp_data_add_tag, gimp_data_remove_tag):
Adapt these to GimpTag being an object instead of a GQuark.

* app/core/core-types.h: Update GimpTag typedef.

* app/core/Makefile.am: Add gimptag.[ch].

svn path=/trunk/; revision=27778