switchbin: Improve caps and accept-caps query handling and add passthrough path functionality

This is a collection of commits that significantly clean up and improve switchbin. Most prominently, the caps and accept-caps queries are now handled better, and passthrough paths are now possible by setting a path's element property to NULL. Such passthrough paths are very useful for postprocessing that is supposed to be applied only for certain caps. For example, path 0 could have PCM postprocessing such as sample rate conversion, and for non-PCM data, path 1 is set up as passthrough.