Now that we can get an accurate HiDPI screen scaling from OSystem,
defaulting to using that seems to make sense. But we may still want
to use a slightly different scaling. The GUI scale option allows
that by providing a scaling (in percentage) with which to multiply
the HiDPI scaling.
I think it works better than a base resolution as it avoids having
the GUI getting bigger or smaller when we resize the window.
This commit keeps a popup widget, but this could be changed with
a slider if we want more fine grain control.