core: Add function to change various core emulator settings on-the-fly

[deltabeard]

A new function should be written to allow the frontend to turn various settings on and off. These may include:

• Ability to switch sound off and on. Useful if the frontend wants to fast-forward without sound.
• Ability to switch certain sound channels off and on.
• Ability to change sound sampling rate. As mentioned in #4, if the frontend detects high load, reducing the sampling rate may help.
• Ability to switch interlaced mode off and on. Drawing half the screen can be skipped to reduce load.