Allow quitting while modal dialogs are open

[adamgoose]

Closes #628

I'm not sure why this limitation was put in place to begin with, and I'm not sure it won't have unintended consequences in other circumstances that I'm not aware of. Feedback welcome.

[mmp]

See #669 for one catch (note: with the same proposed fix). However, retesting this now, everything seems to work as expected even with fatal error dialog boxes. Could you check as well on your system, adding something like ShowFatalErrorDialog(render, plat, lg, "oh no") right before the for { loop at the bottom of main.go and confirm that the dialog is still shown/vice doesn't exit immediately in that case?

[adamgoose]

Yes, it appears to stay open. Not until I click "Ok" does the process terminate.

[adamgoose]

However, it looks like that's an insufficient test. While trying other airports, I'm getting actual crashes, and the fatal error dialog is only visible without this change.

[adamgoose]

I take it back. The error I was seeing was related to #710, which does not show a Fatal Error dialog with or without this change. Call it an unrelated red herring.

Funny enough, the ShowFatalErrorDialog(render, plat, lg, "oh no") appears to prevent Cmd + Q from actually quitting the program. However, I can Cmd + Q when the "Create Scenario" dialog is open.