Summary:
This patch extends commit1a47ae24 and
addresses the following issues:

1) The state of the menu action checkbox was not updated when the other
   window changed the its state
2) There should be **no** access to KisConfig in the OpenGL rendering
   loop (it is slow due to possible disk access).
3) A canvas update call should be issued right after the config changed,
   otherwise the old state of the grid will be visible until one hovers
   the canvas

BUG: 389033

Test Plan:
1) Open multiple Krita windows and zoom both of them to 1000%
2) Change View->Show Pixel Grid in one of the windows
3) The state on the checkbox and the canvas should automatically
   update in the other window

Reviewers: #krita, scottpetrovic

Differential Revision: https://phabricator.kde.org/D10171