w32_common: prevent display power management

Handling SC_MONITORPOWER doesn't seem to prevent the screen from
dimming. The recommended way to do this in Windows XP and Vista is to
call SetThreadExecutionState with ES_DISPLAY_REQUIRED. Windows 7 also
has the PowerCreateRequest/PowerSetRequest/PowerClearRequest APIs but
they're probably too complicated for this task.
Merge pull request #124 from rossy2401/w32-display-required

Prevent display power management in Windows
input: don't ignore press-only mouse button events

Before this commit, only mouse events with both down and up events
were processed. This caused a regression with ignoring mouse wheel
events in cocoa, because these don't distinguish between up and down.

Regression caused by 5b38a52.
demux_mkv: pass extradata for opus

Fixes playing 5.1 opus audio tracks.
Add some kind of IPC mechanism using zeromq

Not ready for public consumption.

Needs --extra-libs="-lzmq" because configure is complicated.
zmq: retrieve values of properties with mp_property_do()

Makes querying volume, position, length, etc. Just Work™, as long as the
client knows what to do with the answer.

Also first try at reporting errors to clients.
zmq: Send error when receiving unrecognised message
zmq: Send update when volume changes
zmq: Send updates when changing properties

Also when adding something to the playlist.
zmq: Replace code with playlist_entry_from_index()

Add support for removing items from the playlist (untested).
zmq: Remove custom "play" command

The new playlist-pos property takes care of this.
zmq: Send playlist-pos when starting to play a file