[rbp] Fix for audio out of sync part 2
[rbp] Apply volume control at audio_mixer when possible
[OSX] Remove unused function Cocoa_GetAppVersion()

When cleaning out the CFStringGetCString references I noticed that
this function isn't used anywhere. So instead of changing it, I rather
just remove it.
Merge pull request #2083 from tru/remove_cocoa_get_app

[OSX] Remove unused function Cocoa_GetAppVersion()
Merge pull request #2081 from popcornmix/mixer_volume

[rbp] Apply volume control at audio_mixer when possible
Merge pull request #2080 from popcornmix/timestamp_fix2

[rbp] Fix for audio out of sync part 2
[OSX] Remove all direct calls to CFStringGetCString and CFStringGetCStringPtr

This adds a new util function DarwinCFStringToString that handles errors
and tries to first get the pointer otherwise copies the string into a
std::string. It also makes sure to use CFStringGetSystemEncoding()
everywhere so we don't get in trouble for using a hardcoded encoding.
Merge pull request #2082 from tru/cfstring_get_cstring_spring_cleaning

[OSX] Remove all direct calls to CFStringGetCString and CFStringGetCStringPtr
[osx] - fix sleep button for nyxboard - use the sleep button key code which will be introduced via PR #2077