CMake: Check CMakeCache.txt for changes

Check CMakeCache.txt for changes and ask whether to apply these to the
project going forward.

This will only consider settings that were previously changed by Qt Creator
and will not pick up on newly changed settings.

Change-Id: Ia20c67bc2a5e9965243f08003c10ec684875387f
Reviewed-by: Tim Jenssen <tim.jenssen@theqtcompany.com>