GtkRange: Fix an event propagation issue

GtkRange: Fix an event propagation issue

The scale in the selectable listbox in page 2 of gtk3-widget-factory
has the flaw that the row is getting selected even when one only
interacts with the scale - unlike e.g. the checkbutton and button
further down in the listbox.

The problem is that GtkRange is trying to claim the sequence for
the drag gesture from the ::pressed handler of a multipress
gesture. Since the drag gesture hasn't seen the sequence at this
point, that is ineffective.

The fix here is to rearrange the gestures so that the multipress
gesture sees the sequence before the drag gesture, and then
claim the sequence from the ::drag-begin handler.