Generic plug-in for relative evdev pointer events.

To use it, launch apps with -plugin EvdevMouse

The plug-in will try to pick a mouse or touchpad device from udev.
If automatic detection does not work, use -plugin
EvdevMouse:/dev/input/eventN to explicitly set the device node.

The initial cursor position is assumed to be (0, 0). Relative events
will generate Qt mouse events with screen positions relative to this
initial position.

Touchpads reporting absolute events will work too, the positions will
be turned into relative. Touchscreens are however not supported.
