Move the initialization of BandLimitedWaves into engine.cpp

Since we now provide the wavetables as pre-generated files, there's no delay caused by their initialization
so we can move it to the startup of the software. I thought engine.cpp is the best place for this, it makes
conceptually more sense than main.cpp IMO.
This way each instrument that wants to use them in the future won't have to call the initialization function
separately, making things a bit easier.