lua : fix tags reporting all images instead of the ones attached to them
lua : gui functions are only available when gui is enabled
allow to pass a lua_State to dt_init

This lua_State will be used if provided.
if set to NULL, a new one will be created instead
lua : fix fomat.write_image not working properly
lua : allow darktable to be a pure lua module

you can now call DT from any lua script without an actual
instance of DT running