GTK: always scroll caret when window size changes

* The old implementation would scroll only once for the file first opened and too early.
  As a result, opening via `sciteco file:number` did not always scroll to the correct position.
  Also, `?` would usually not scroll the topic into view.
* Instead, we now always scroll in all views, but only if the size allocation changed.
  This ensures, we can still scroll with the mouse wheel.
* Therefore, we have to store the current size allocation per view.
  Instead of allocating a separate heap object, I decided to subclass the Scintilla GTK class.
* Some explicit casts are still necessary since teco_view_t is typedefed
  to `struct teco_view_t`, but we cannot easily rename the GObject instance structure.

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