Add hot corners to the top, bottom, left and right sides

Images are not right though.
Set minimum Qt version required
Detect KScreen
Link greenisland to Qt5 targets

We don't need qt5_use_modules, that is needed for CMake 2.8.9 support
but we are already requiring a newer version.
Use KScreen instead of our own solution

KScreen is better than QScreen as it's capable of detecting when primary
screen changes and has a nice fake backend.
We'll need to add Wayland support to KScreen though.

The initialization sequence was changed a little bit to get the screen
estate in advance and change compositor output geometry.
Also, the screen model is now created from main().