fix press/release effects of the mute switch

fix #20 (closed) and #30 (closed)

I tried the naive solution which consists of forwarding the Gdk events to the GtkSwitch, but it won't work (not sure why... possibly because the implementation of GtkSwitch uses its own GdkWindow...).

So the solution implemented here decorrelates the active state and underlying state of the GtkSwitch. Button press only changes the active state, the underlying state is changed only on button release (and the active state is reset when the pointer leaves the window).