fix PixFader size allocation

previously faders in the processor box were only sized correctly
after the 2nd iteration: allocate -> request -> allocate
processor drag box only
continued work on processor-routing visualization

major re-design.
 * change splitting-icon into routing-icon
 * add it to all processors (not only plugin-inserts)
 * subscribe to ChanCount configuration changes
 * add 'wiring' to BlankProcessorEntry pre-fader placeholder
 * visualize routing for non-matching port-counts

currently still wire+color design with #if'ed debug messages
and optional #ifdef wires for matching connections.
fix ardour-button text bleed