i3 config: Hookup XF86TouchpadToggle to my wmii trackpad plugin

XF86TouchpadToggle will toggle the "SynPS/2 Synaptics TouchPad"
Shift+XF86TouchpadToggle will toggle the "TPPS/2 IBM TrackPoint"

I don't know what kind of variations there are in those names - let me
know if you have a trackpad or other input device you want to toggle
that has a different name with:

$ xinput --list