Split OS X launcher into 2.8 and master versions

GIMP launcher for GIMP 2.8 and master differ in the GEGL path. Otherwise
GIMP master on OS X will not start due to unfound GEGL.
Add missing language files to OS X bundler

Fix typo (filename.mo). Add missing language files for GIMP and its
dependencies. Prepare translation of OS X application menu strings.
OS X build: smaller fixes and cleanups

README: Fix missing information. Formatting
gimp.modules: Add Github. Switch qtk-quartz-engine repo type. Disable
gtk-doc generation for glib.