core: add GRL_METADATA_KEY_MB_RECORDING_ID

MusicBrainz recording identifier. From its documentation:
A recording is an entity in MusicBrainz which can be linked to tracks
on releases. Each track must always be associated with a single
recording, but a recording can be linked to any number of tracks.

https://bugzilla.gnome.org/show_bug.cgi?id=741230
core: add GRL_METADATA_KEY_EPISODE_TITLE

Title of show's episode.

https://bugzilla.gnome.org/show_bug.cgi?id=741207
core: Allow removing metadata keys in GrlData

https://bugzilla.gnome.org/show_bug.cgi?id=741207
core: Fix API documentation bugs

data/grl-media-audio.c:180: Warning: Grl: grl_media_audio_set_mb_recording_id: unknown parameter 'mb_track_id' in documentation comment, should be 'mb_recording_id'
data/grl-media-video.c:182: Warning: Grl: grl_media_video_set_episode_title: unknown parameter 'show' in documentation comment, should be 'episode_title'
build-sys: fix gio-2.0 gir bindings errors

Use vala gio-2.0 definition to avoid the gir error:

Gio-2.0.gir:79247.7-79251.24: error: `UnixSocketAddress' already
contains a definition for `abstract'
Gio-2.0.gir:79093.7-79095.21: note: previous definition of `abstract'
was here

https://bugzilla.gnome.org/show_bug.cgi?id=741315
build-sys: fix out-of-tree build of vapi

Reference custom file by path.

https://bugzilla.gnome.org/show_bug.cgi?id=741315