Efl.Gfx.Filter: Rework APIs and mark as stable

Efl.Gfx.Filter: Rework APIs and mark as stable

- Remove @beta flags,
- Update @since to match stabilization,
- Change methods to properties with keys,
- Use eo_prefix and add filter_ prefix to all properties since
  they use very generic names,

The filter API stays under Efl.Gfx since there are other kinds of
filters, and this one is the particular "graphical filter" or
"effect" API.

The EO API mostly not change from an application point of view,
except for "source_get" which now returns a string directly. Also,
state and data can now be queried.