macOS support

[bmwalters]

TODO

• local_events.txt, global_events.txt and Custom Resources attempt to load from wrong file paths
• keybinds should use cmd instead of ctrl
• crash when opening midi chart due to "import vocal track?" dialog being presented on background thread
• CH export says files already exist when song is untitled
• export ch package -> export -> choose folder -> override? dialog
  • upon hitting cancel, this error is spammed
• export ch package dialog -> export button -> file browser
  • create a folder "aeiou"
  • "e" keyboard event is sent to game even though it is blocked and "extended sustains" is toggled
• menu bar dropdowns open up on hover sometimes
• after a little bit there's extremely high CPU usage. makes my dual core macbook air scream.
  Settings > Editor > Target FPS > 60 and disabling AA fixed this.

Details

• Added macOS implementation of native window, file explorer, and message box
  • These implementations are written from scratch in Swift. The Xcode project is included.
• Uses the FFmpeg OGG transcoding solution written for Linux on macOS as well.
• Added BASS audio libraries for macOS
• Added build command for macOS

Fixes #16

[FireFox2000000]

@bmwalters
Cmd keybind can be added into KeyboardDevice.cs under the ctrlKeyBeingPressed property and implemented into the ToModifierKey function, which should handle all input changes without having to rebind the default controls. Might need to change the display string for it as well which is generated under KeyboardMap.cs in GetInputStr.

[FireFox2000000]

Note that Opus audio compatibility has been recently added and this will also need separate implementation here as well. Eventually will add the ability to export this audio format as well, using bass' opus audio encoder.