Start using the new wl_fixed_t type properly

Start using the new wl_fixed_t type properly

The mouse and touch coordinates cannot just be treated as integers
anymore, they need to be converted from/to double using the helper
functions. Some necessary QPoint -> QPointF changes have also been
made.

For the Qt-specific touch extension we will not switch to wl_fixed_t
though. This is because the precision is unfortunately quite small
(factor of 256 vs. 10000).

Change-Id: I23deaaffe478a39495b12d336985bc62e38a6af4
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>