Add more padding to the contents of applet configuration

For some time now I haven't been able to shake the feeling that the contents of applet configuration don't have enough room to breathe.

So in this patch I added a little bit of padding from all sides (smallSpacing).

If this is accepted, it would require a companion patch for QWidgets configuration windows.

-- Before:

After:

-- (Sidenote: the buttons footer is misaligned, but it's best to solve that in a different patch)

cc @teams/vdg