Fixes a bunch of errors:
- libedit and libbrowser would zoom to bounding box but centre on canvas
- libedit, libbrowser and gerbview didn’t take the scroll bars into account
- pcbnew didn’t take scroll bars into account or apply the 10% margin
- appending a board file would re-centre, but not re-zoom
Fixes: lp:1504302
* https://bugs.launchpad.net/kicad/+bug/1504302