We were running into various corner conditions where a tool's event
loop would exit while the tool was still active, or the tool would
get popped while we were still in the event loop. (A lot of these
had to do with the POINT_EDITOR's, but not all of them.)
The new architecture:
1) tools always do a Push()/Pop()
2) everyone is responsible for their own pops; no more stack-clearing
on a cancel
3) CancelInteractive events go to all tools to facilitate (2)
In singletop mode, all frames show the "Quit" option in the file menu
and will quit on Ctrl-Q. When launched from the main KiCad interface,
sub-programs show the "Close" option instead and will close with Ctrl-W.
In this mode, Ctrl-Q will instruct the main program to exit.
Fixes: lp:1779938
* https://bugs.launchpad.net/kicad/+bug/1779938
