[Plugin GStreamer] Add early discovery of audio stream
⋅ And add some reformating
[Plugin GStreamer] Rewrite cleanly discovery function
[Context] Revert useless function
[Playlist] Factorise play function
[Plugin] Rename myp_plugin_status_t to myp_plugin_state
[Plugin] Export discovery into the API
[UI Plugin] Add interface and Gtk implementation
⋅ Add UI-plugin Gtk
⋅ All plugins should be as independent as possible, however, for
 efficiency reasons, multimedia plugin needs to access some
 fonctionnalities like window opening, overlaying, closing and sizing.
⋅ common.h is used to share data (only enum myp_plugin_state_t for now)
 between UI plugin and multimedia plugin.