Drop all uses of wxRAISED_BORDER and wxSUNKEN_BORDER from dialogs and
panels used in main frame windows.
Drop use of wxSP_3DSASH options on all wxSplitterWindow definitions.
Minor dialog alignment and spacing fixes.
- 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