Implemented an Interpolator class.

Moved Linear interp. and added Cubic and Lanczos Interpolators.

AudioCommon.cpp/h: added function to retrieve algorithm strings

Interpolator.cpp/h: contains base Interpolator class, as well as three children - Linear, Cubic and Lanczos interpolators

Mixer.cpp/h: moved CMixer::MixerFifo::MixerFifo constructor to .cpp, along with creation of the interpolator based on settings; moved Linear interpolator away; added call to function to populate floats for Cubic and Lanczos interps.

ConfigMain, ConfigManager: add defines for the algorithm names, and added wxChoice in DSP settings in Backend box for the interpolation algorithm. I guess my MSVC didn't like some of the spaces besides the |


`GetInterpolatorAlgorithms()`

Interp is easy to misinterpret for anyone not used to the codebase.