add prefix to mouse button state enum values
implement rtb_mouse_event.click_count
renumber/rename some system events

RTB_MOUSE_DBLCLICK has been made redundant with RTB_MOUSE_CLICK +
rtb_mouse_event.click_count, and RTB_DRAGGING becomes RTB_DRAG_MOTION
x11-xcb: rtb__mouse_double_click_interval()
knob: double-click to reset to origin

also, remove/clean up some of the other interactions.
implement RTB_MOUSE_WHEEL events
knob: change value on RTB_MOUSE_WHEEL events