WPrefs: reorganised the Window Handling Settings panel for better look
Due to a long history of evolution, it was time to rethink the placement
of all widgets to achieve a clean look. The frames have been reshuffled to
be aligned to each other, and as a bonus the Mod+Wheel setting now has its
own frame instead of being part of another unrelated frame.
Took opportunity to include an info balloon to explain what the "by
keyboard" checkbox actually means.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>