add support for play files opened from external ('open with' or double click)

[aargirakis]

What should happen? Add file to what playlist? Start playing it? etc.

[ciros88]

if #139 preference is enabled then add the track to the current selected/used playlist.

if #139 is disabled or not implemented then open a new bzr2 instance adding the track:

1) on the default playlist or 2) to a new one if default playlist isn't the current selected/used but others exists

choose the behaviour you prefer: indeed 1 is more simple and probably better to choose as 2 needs to decide a strategy for playlists behaviour and should be done in another issue.

in any of these cases, the added track should be played replacing current ones or enqueued depending on #139 checkbox "enqueue items"

[ciros88]

2.0.44.Alpha:

• playback of m3u playlists opened from external action is not working
• enqueue (from #139) not handled yet

[ciros88]

2.0.50.Alpha: playback of m3u playlists opened from external action works, but start playing from the the last playlist track

[aargirakis]

Yes, and what do you think should happen? Playing the first track in the clicked playlist?

[ciros88]

2.0.60.alpha: when launch a m3u/m3u8 playlist from the external it starts playing from the last m3u/m3u8 track instead of the first

here the detailed behavior to use when launching files from the external.

1. one instance mode enabled:

   • the launched track file should be added as the last entry of the selected playlist (the single clicked one) also if it is not the currently playing playlist (the double clicked one) so people can listen some music in a playlist while organizing another playlist.
   • the launched m3u/m3u8 file should be added as the last entry of the selected playlist (the single clicked one)

   1.1. enqueue items is disabled:

   • the launched track file should be played
   • the launched m3u/m3u8 file should be played starting from its first track

   1.2. enqueue items is enabled:

   • with both launched track or m3u/m3u8 files the current playback shouldn't change

2. one instance mode disabled:

   • the launched track file should be added to the default playlist (because the meaning of "default" is also this) and then played (since there is no enqueue in one instance mode)
   • the launched m3u/m3u8 file should be added to default playlist and then played starting from its first track