Add scaling adjustment for touch dragging a scrolled window

As described in this VTE issue the GtkAdjustment is supposed to abstract away the scale of the GtkScrollable such that the scrollable can for example use the number of rows scrolled instead of a pixel value. This PR makes touch input work for those different scales, otherwise every pixel dragged by your finger moves a whole row in a terminal emulator like KGX.

There's a similar issue with kinetic scrolling in VTE that may be related to this but I don't currently have a touchpad to easily test this on: vte#234 (closed)

CC @carlosg