Make some "GstPlayFlags" settings as part of the API

[Rafostar]

With current GstPlayer API we have access to changing the GstPlayFlags of the playbin(3) and we do allow changing a few of them. The problem is that GstPlayer is also changing some of them (e.g. video/audio/text to en/disable tracks).

Clapper should provide an API that only allows users to change the ones that aren't changed internally via other settings, otherwise things are gonna get broken. Also worth remembering is that flags set by default in playbin2 are different in playbin3.