xfce4-settings: Circular scrolling.

[Peter de Ridder]

Hi devs,

I created a feature branch for xfce4-settings. "peter/circularscrolling".
This branch includes an addition to the mouse settings to enable
circular scrolling.
Since this would give 4 options for scrolling I also changed the UI
from radio buttons to a combo box.

The technical implementation is a followed:
If circular scrolling is selected and horizontal scrolling isn't. The
synpatics settings "Synaptics Circular Scrolling" is set to 1 and
"Synaptics Circular Scrolling Trigger" is set to 0 (any edge).
If circular scrolling and horizontal scrolling is selected. The
synpatics settings "Synaptics Circular Scrolling" is set to 1,
"Synaptics Circular Scrolling Trigger" is set to 3 (right edge) and
"Synaptics Edge Scrolling" is set to [0 1 0] (Only horizontal
scrolling).

Does anyone have objections if I merge this into trunk?
I'm planning to merge this on Thursday.

Regards,
Peter