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.~ The frontend should handle this, not Peanut-GB which doesn't do APU emulation.
• ~Ability to switch certain sound channels off and on.~ The APU should do this (not included in Peanut-GB).
• ~Ability to change sound sampling rate. As mentioned in #4, if the frontend detects high load, reducing the sampling rate may help.~ Another item for an APU to implement.
• Ability to switch interlaced mode off and on. Drawing half the screen can be skipped to reduce load.

[deltabeard]

Interlaced mode was added a very long time ago. The other items are related to the APU, which Peanut-GB doesn't have integrated.