[Merge] lp:~phablet-team/mir/track-plug-state into lp:mir

[Andreas Pokorny]

oh there are really only three diplays possible HWC. builtin external and virtual. So this could just be an int. Instead of an array.

I guess we should integrate that to 0.20?
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https://code.launchpad.net/~phablet-team/mir/track-plug-state/+merge/285854
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