OSX: Renamed Mac project to OSX. Added Bass.Net.OSX.dll.

- "_Mac" has been replaced by "_OSX" to match the OS name change. It
could have been named Cocoa (like MPfm.WPF and MPfm.GTK) but since Cocoa
is exclusive to OSX, the name OSX has been chosen (like .iOS and
.Android).
- Added a different version of Bass.Net for OSX to access CoreAudio
methods. This unfortunately required adding several new .csproj files.
- Bassenc doesn't seem to be loaded like a decoding plugin, removed code
to load the plugin in startup sequence.