Scrolled Viewport: use viewport_motion_handler as the only timeout indicator

The boolean viewport_motion is removed in favor of directly using the source ID. Whenever the timeout callback function decides to return FALSE and stop further repeating timeout callbacks, set the source ID to zero to avoid cancelling a timeout that already vanished.

The previous find_source_by_funcs_user_data is insufficient to prevent this state discrepancy between glib and viewport struct and also prevents scrolling from actually happening, so it's removed.