AFP: Add interpolation selector, fix segfault-causing bug with reverse m...

...ode

Bug with reverse mode fixed - Closes #620 

The user can now choose from three interpolation modes: none (zero-order hold), linear and sinc (uses medium-quality always). I included none because the lo-fi sound may be desirable in some cases (eg. chiptunes and similar retro stuff). This also provides us with the groundwork and UI architecture to add pitch-shifting sample playback mode (using librubberband or similar) in the future if we so wish.