evdev tablet support on Linux

Nowadays on at least some distros XInput presents the Wacom and other
tablets with the TABLET atom instead of separate STYLUS and ERASER.

Task-number: QTBUG-25329
Change-Id: I24d86be63ac2ffdeeb042d91644610feb6c9f4e7
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Andy Shaw <andy.shaw@digia.com>