Add Tablet Support
This adds basic drawing tablet support. This is based on old patches that where sent to the mailing list some time ago.
I've changed quite a bit of the code to adapt do various changes that happened in the Weston codebase since the last version was posted. And I improved and fixed things all over the place.
The new v2 version of the protocol has been released in the meantime so I've updated the code to use it. But I've not implemented the new 'pad' API. But this should be already quite useful without it.
And I squashed several of the patches to make the branch bisectable.