OS X build: fix GVFS dependency

Update glib, glib-networking, shared-mime-info, libsoup to latest
versions. Re-enable building GIMP with GVFS. Fix formatting.
OS X build: replace 2.8.8 target by gimp-osx target

Remove the outdated gimp-2.8.8 target, add target 'gimp-osx' for testing
the changes together.