app: handle migration on OSX of GIMP 2.8.2's config folder.

GIMP 2.8.2's config was "~/Library/Gimp/x.y", before it got moved to
"~/Library/Application Support/x.y" and after being saved in the home
like other UNIXes. The migration code will now check all 3 places in the
right order on OSX.
Thanks to Simone Karin Lehmann for the original proposition.
libgimpthumb: new function gimp_thumb_get_thumb_base_dir()

Also use this new function in the gimp-thumbnail-list tool, which must
always use the same thumbnail folder as GIMP core.
libgimpthumb: adds "Since: GIMP 2.10" information to gimp_thumb_get_thumb_base_dir()
libgimpthumb: fix "assignment discards ‘const’ qualifier" warning.