Merge AudioSubsystem into USurrealAudioDevice

[LupertEverett]

This PR moves ALL functionality in AudioSubsystem to USurrealAudioDevice. This both unifies the AudioSubsystems, and makes the sound and music sliders in settings finally work (the issue was the settings were saved to USurrealAudioDevice, which did pretty much nothing; and AudioSubsystem was actually handling all the sound playing stuff)

Some details about the movearound:

• distSquared(vec3, vec3) function in AudioSubsystem.cpp is now moved to vec.h file, where it makes more sense
• USurrealAudioDevice has InitDevice() and ShutdownDevice() functions to handle the initialization and destruction of the device. The latter was added because SurrealEngine would crash during shut down otherwise.

Known bugs:

• Turning down the sound volume will also turn down the music volume. I've no idea why that happens.