Background Music v0.4.0

Changes since 0.4.0-SNAPSHOT-c0ab98b:

 - Added app volume and pan settings to the AppleScript API. Thanks to
   @marcuswu (kyleneideck#457).
 - Add a bundle ID property for apps in the AppleScript API.
 - Installer: Retry if opening BGMApp fails.
 - Fix uninstall failing when BGMApp has debug symbols.

Changes since 0.4.0-SNAPSHOT-f24a6cb:

 - Workaround deadlocks when starting IO on macOS Big Sur.

Debug build at commit f20dd57

Debug build at commit 026558d

Debug build at commit ae724fe

Debug build at commit ae564d7

Debug build at commit 42fbd75

Debug build at commit 6edf25b

Changes since 0.4.0-SNAPSHOT-70a64de:

 - Try to avoid using the discrete GPU on multi-GPU systems, which
   wastes energy/battery.