Merge the rest of the tagging code developed on the tagging branch
by Aurimas Juška. Development will now continue in trunk.

* app/core/gimptag.[ch]: New files (not strictly true but almost)
implementing the represention of a tag.

* app/core/gimptagcache.[ch]: New files implementing functionality
for loading and saving tags to tags.xml, and assigning loaded tags
to tagged objects.

* app/core/gimpfilteredcontainer.[ch]: New files implementing a
tag filtered GimpContainer.

* app/widgets/gimptagentry.[ch]: New files implementing a
GtkEntry-like widget for entering tags.

* app/widgets/gimpcombotagentry.[ch]: New files implementing a
combobox-like widget for selecting tags.

* app/widgets/gimptagpopup.[ch]: New files implementing a popup of
all available tags that can be selected and combined in a
checkbox-like way.

* app/core/gimp.[ch]: Add a GimpTagCache member and manage tag
assignment and saving and loading to/from tags.xml.

* app/widgets/...