Cinnamon settings: Add touchpad settings to the mouse panel

Addresses #1930 


Reworked the mouse panel to use a tabbed approach - mouse and touchpad.

Also added ability to size panels to a specific height in cases where we can't automatically retrieve a good preferred height for the content box (as in the case of the mouse panel)

![screenshot_area_2013-04-29_19 13 27](https://f.cloud.github.com/assets/262776/441694/b259721a-b122-11e2-803b-475aa01a864c.png)