Remove some useless spacers, and do not force some widgets to have a to
large size (by removing empty areas).
Work in progress: one panel is still too big.
You can now choose the behavior of dragging with the
middle and right mouse buttons.
You can also choose which modifier keys to use for
panning and zooming with the scroll wheel or trackpad.
You can also customize the zoom speed, which makes
it possible to have a good zoom experience on a wider
range of input devices.
You can also now zoom by dragging with the right or
middle button if desired.
Fixes https://gitlab.com/kicad/code/kicad/-/issues/3885
Fixes https://gitlab.com/kicad/code/kicad/-/issues/4348