Fixup #12984 for isInMessageBox backwards compatibility (#13011)
Fix up of #12984
Summary of the issue:
Backwards compatibility for the following code was broken in 2021.3 beta.
from gui import isInMessageBox
Note: this code creates a copy of the value of isInMessageBox, and as such, was only accurate if it was being imported when it was being checked.
Importing like this also prevents gui.isInMessageBox from being updated:
from gui import isInMessageBox
isInMessageBox = False # this doesn't update gui.isInMessageBox
Description of how this pull request fixes the issue:
Creates an isInMessageBox that is updated by isInMessageBox.
Note that while from gui import isInMessageBox can run without error now, using or updating the variable will not work as expected, nor has it ever worked properly.
