Partial merge of code from Aurimas Juška.

* app/core/gimpdata.c: Implement GimpTagged::get_identifier() and
GimpTagged::get_checksum().

* app/core/gimpbrush.c: 
* app/core/gimpgradient.c
* app/core/gimppalette.c
* app/core/gimppattern.c: Implement GimpTagged::get_checksum().
They all use the GimpData implementation of
GimpTagged::get_identifier().

svn path=/trunk/; revision=27784