examples: add an example for a fake tablet

Testing tablets without acces to the hw is reasonably common, let's provide a simple example that can be extended to the user's liking.

cc @linkmauve

run with sudo examples/fake-tablet.py. It's a very basic script but shouldn't be too hard to figure out how to extend this to your liking. change the pressure to fake tip down/up events, use BTN_STYLUS to simulate a pen button click. Change to BTN_TOOL_RUBBER as noted in the code to test the eraser end.