EvdevTouch: Added "invertx" and "inverty" parameters

In some cases the event axises reported by evdev are inverted.
This commit adds plugin parameters invertx and inverty to
invert X- and Y-axis.

Change-Id: Idaa63affd8321aa47974788c32d978fc21eb3dec
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>