Add a new server option "touchInputLocal" that keeps touchscreen/pen
input on the server computer even when the cursor is on a client screen.

This is useful for devices like the Surface Book where you want to use
touch to interact with local applications while the mouse cursor is
controlling a different computer.

Implementation:
- Detect touch-generated mouse events in the low-level mouse hook by
  checking dwExtraInfo for the touch signature (0xFF515700)
- When enabled and cursor is on client, let touch events work locally
  but don't forward them to the client
- Add GUI checkbox "Keep touch input on this computer" in server config
- Add config file option "touchInputLocal = true/false"