gtkscrolledwindow: Consider shift key presses when decelerating [gtk3]

This is the gtk3 version of !286 (merged). It does not change the code in gtk_scrolled_window_scroll_event which would be the most comparable to the code changed in the gtk4 version of the patch, because the velocities returned by scroll_history_finish already include the scroll unit sizes which depend on the direction that is being scrolled. It would also be possible to move those out into gtk_scrolled_window_scroll_event and have a (slightly bigger) patch that is more similar to the gtk4 version if you prefer that.