Directly set backend font options in clutter_set_font_flags

Directly set backend font options in clutter_set_font_flags

Instead of having a separate set of font options that override the
backend options when clutter_set_font_flags is called, it now just
directly sets the backend font options. So now the font flags are just
a convenience wrapper around the backend font options.

This also makes the ClutterText labels automatically update when the
font flags are changed because they will respond to the 'font-changed'
signal from the backend.