Party Mode option

[lukedom89]

Added option to enable a squeezer party mode. This protects the current playlist and players from modification and restricts access to all settings.

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This change is 

[nikclayton]

Hi,

Thanks for the pull request. Just started experimenting with this, so some initial comments:

• I'm not completely sold on this being in "Settings". I wonder if it would be better as a toggle-able entry on the action bar menu.
• Rather than keep the UI elements active when party mode is enabled, and display a toast when the user tries to do anything with them, perhaps it would be better to hide / disable the affected UI controls when party mode is active. If I'm following this properly, that would basically result in just (some parts of) NowPlayingActivity and CurrentPlaylistActivity being available (as well as settings). This would provide a good visual indication in the UI that "something different is happening". It also keeps SqueezeService unaware of party mode, which feels like a good thing, as far as separation of concerns goes.
• Could you jot down a specific list of what this is / is not supposed to allow, so we can (theoretically) add tests for it, and make sure that future functionality doesn't break it? From what I can tell, the intent is to allow the user to (a) see what the current song is (and position), and (b) see the upcoming playlist. But any other modification (position in song, volume, position in playlist, etc) is supposed to be forbidden? If that is the case then the functionality is incomplete -- with party mode enabled you can go to the HomeActivity, and choose a new random mix for example.

Hope that's helpful -- Nik