GTK: handle (smooth) scrolling with a scroll controller

* On interfaces, which only support smooth scrolling, we had to emulate
  discrete events. Moreover, once we enabled GDK_SMOOTH_SCROLL_MASK,
  systems that would previously report discrete UP/DOWN events, would
  suddenly report GDK_SCROLL_SMOOTH.
* Converting from smooth scroll events to discrete scroll events
  turned out to be trickier than anticipated.
  Scrolling was therefore more sluggish than it used to be before
  2f448c976889fe60aba8557b5aa4aa0a0d939281.
* Scrolling is therefore now delegated to a GtkEventControllerScroll,
  which is used to synthesize a discrete GDK_SCROLL event that's
  fed into the usual event pipeline via teco_interface_input_cb().

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