I've stripped out everything not strictly required for mouse button HID
support. This include dropping the worker thread completely, shaving
down the USB HID descriptor. Flattening the mouse/ source directory
structure and removing a bunch of event handling.
I have kept the mouse event handling separate from the other HID event
handling since I figured that was a pretty neat split. If that's a bad
idea, do tell.
I've also added a test case for mouse button emulation, since that was
untested before.
The changes have been tested on a corne (split) in usb mode. Bindings on
both the left and the right side works (with the left side as master).
