Basic VST plugin support

This patch series adds support for VST plugins.  The user can load one or more plugins into an effects chain.  For example, a software amplifier followed by a reverb.

To use this feature, select "File" | "Configure VST..." from the menu.

Tested the following native Windows plugins: S-Gear 2, GuitarRig 5, and Amplitube 3.  Also tested native Linux plugins.

I do not have access to the Steinberg VST SDK or docs due to its GPL-incompatible license.  Hopefully my host implementation is reasonably correct.

Known issues with LePou's LeCab2 (crash).  Please report bugs if plugins crash.

Limitations:
 * VSTi instruments are not supported because Wahjam currently lacks MIDI support.
 * Mono only because Wahjam does not support stereo channels yet.