Use standard Magnetize() to handle grid and magnetic pads.
Use the standard menu from PCB_TOOL. Delegating the menu to
the SELECTION_TOOL just caused grief.
Also brings clients (such as Position Relative To) into line,
and implements better Cancel behaviour.
Also improves visibility of modal status messages:
- moves Select Anchor message from status bar to popup
- moves Select Reference message from dialog to popup
Fixes: lp:1786727
* https://bugs.launchpad.net/kicad/+bug/1786727