A new items is added to the GAL display options (and the dialog), and a
hotkey (Ctrl+Shift+x) is added to toggle it.
Fixes: lp:1673633
* https://bugs.launchpad.net/kicad/+bug/1673633
BOARD::GetBoundingBox() now directly calls BOARD::ComputeBoundingBox()
and there is a new method BOARD::GetBoardEdgesBoundingBox() used for
call sites that needed to use ComputeBoundingBox( true ) in the past.
This allows COMMON_TOOLS to implement ZoomFitScreen without knowledge
of the BOARD class.
Some grid/zoom tools are left in PCBNEW_CONTROL because they currently
depend on Pcbnew-specific class members. Once refactoring is done to
make it possible to use all zoom and grid controls outside of pcbnew,
these last tools can be moved to common to match their ACTIONs.