Also fix a bug where resetting the values doesn't work if
the thickness units were changed because it tries to keep
the old width values which are then re-scaled to the reset
thickness units. Probably better to just reset the widths
too, as it's potentially less confusing.
On wxWidgets 3.0, SetSizeHints must be called once the panel is fully populated.
On wxWidgets 3.1.5, SetSizeHints is not mandatory but can be called to ensure
right sizes once the panel is fully populated.