Touchpad: add tap-and-drag setting

[cassidyjames]

Fixes #184.

• Switch tap_to_click switch to check
• Add tap_and_drag check
• Bind its sensitivity to the tap_to_click check
• Implicit GLib namespacing for binding flags while we're here

[cassidyjames]

This is where I am now. ~For some reason binding tap-and-drag sensitivity to the tap-to-click setting isn't working but I haven't revisited that; I'll do it now.~

I don't know how I feel about the layout. I like Tap-to-click being a switch rather than both being checkboxes, but we could do something like this:

Thoughts @elementary/ux?

[danirabbit]

@cassidyjames hm yeah the switch feels weird. I'm more in favor of the two checkboxes personally