dBTP: re-send peak values to UI on re-open

Add a simple handshake using control-ports...
hack for LV2-hosts that don't support ui:protocol ui:peakProtocol
tweak goniometer auto-gain (add rms, target level)
spectr: re-scale attack/decay - prepare for 0..100 dial
fix needle-meter screw (X/Y drag distance)
add dial widget, use in spectr
extend dial-widget and use it in the goniometer
add shaded pattern to dial widget
make dial widget react on mouse-scroll events
add labels to dials