video filter - general improvements

The playback service now provide a single accessor to control the video filter properly.

This transfer the responsibility from the view to the service to :

• access to all filter parameters' value, default value and allowed values range
• reset the video filter if needed

The view also now disable the filter properly when a reset is requested.

Those changes also fix another issues where filter was always reset when playback view was minimised or closed.

It needs VLCKit!128 (merged) to build.

It will be marked as ready when a new MobileVLCKit version is released with VLCKit!128 (merged) approved.

Closes #1240 (closed)