This implements the pad import/export to the board's master pad setting
in the GAL canvases.
Implemented as a new GAL tool: PAD_TOOL.
It uses the same dialog, which has been split out into its own files in
pcbnew/dialogs, rather than along with frame methods in
pcbnew/globaleditpad.cpp.
Fixes: lp:1619304
* https://bugs.launchpad.net/kicad/+bug/1619304
John Beard