System sounds are UI-related sounds like mail notifications or terminal
bells.
Xcode 9.2 doesn't support saving .xib files in Xcode 7 format any more,
so building Background Music now requires Xcode 8 or above.
Also, fix some of the tooltips that would only work if BGMApp was the
foreground app, which it shouldn't be.
BGMApp now disables BGMDevice's volume and/or mute controls if the
output device selected in BGMApp doesn't have matching controls. This
prevents the controls from being presented to the user when they don't
do anything.
In BGMPlayThrough, wait much longer for our IOProcs to stop themselves
before assuming something's gone wrong. In testing, rapidly changing
between output devices with and without controls while playing audio
would occasionally cause one of the IOProcs to take too long to stop
itself.
Also adds some basic scriptability, mainly so UI tests can use
AppleScript to check BGMApp's state that would be complicated to check
otherwise. (In this case, to check which output device is selected.)
Fixes#101.
The BGMApp project now builds an XPC service bundle called BGMXPCHelper,
which vends a Mach service that BGMApp and BGMDriver can use to
communicate. This will hopefully be useful for some of the tasks HAL
notifications aren't suited to.
In this commit, BGMDriver uses the XPC helper when starting IO, to wait
until BGMApp is ready for playthrough. BGMApp can only start playthrough
when the output hardware is ready for IO. BGMDriver can now tell the HAL
when we're ready for IO, which means we don't have to keep the output
hardware running all the time (or drop frames or increase latency).
The end result is that playthrough doesn't waste CPU time while idle any
more. This also means that now playthrough won't prevent the system from
sleeping when idle.
