This ensures the colors used by the status text pane in the preview
widget are the same as the canvas, so there is a seemless transition
between the two.
Additionally, remove the scrollbars from the symbol preview widget
in the place symbol dialog - they are pointless.
- Pull out compound widget bits into FOOTPRINT_PREVIEW_WIDGET
- Move all pcbnew-specific bits *inside* pcbnew; implementation should
be private for users
- Make a few class members and inner types private