Partial merge of code from Aurimas Juška.

* app/core/gimpdata.[ch] (gimp_data_make_internal): Add an
'identifier' parameter/instance struct member that can be used to
identify the internal GimpData object across sessions. It is the
internal-object counterpart to a file path.

* app/core/gimp.c
* app/core/gimpcurve.c
* app/core/gimpbrush.c
* app/core/gimppattern.c
* app/core/gimppalette.c
* app/core/gimpgradient.c
* app/core/gimp-gradients.c: Assign an identifier to the the
internal GimpData objects.

svn path=/trunk/; revision=27781