Minor audio additions

These are a few various nice-to-haves, the commit message explains what they do fairly well.

Probably the only debatable change is that `Param::getValueArray()` is no longer const - Because now it will always return a valid array of values (even if they are all the same value), the function may have to call a `dsp::fill` on the internal buffer. This is nice in the case of an `audio::Node` that has more than a couple Param's, since it really simplifies the logic at a minimal, pretty much negligable, performance penalty.  At the same time, `getValueArray()` should almost always be called from a non-const function (such as `Node::processs()`, due to the block-based nature of audio processing where state has to be stored between processing blocks.

These changes are fairly well tested, cherry picked from my private dev branch.

This should be merged into both *master* and *glNext* branches.