Qtfred error checker

[MjnMixael]

Activates and greatly enhances the error checker in QtFRED. Instead of just one popup per issue causing an annoying stream of popups this version aggregates all errors and shows them in a single dialog with various error levels. It also tries not to silently modify mission data but has a feature to automatically fix simple problems, changing unclear implicit behavior to explicit.

Part of getting this to work is silencing mission errors during parse so that QtFRED can rely on the error checker to handle that more gracefully. Error Checker here explicitly does not handle mod data related errors so those popups remain. So I've added a QtFred_running variable here so that FRED2's behavior is unaffected. Once FRED2 is fully deprecated, QtFred_running can be merged with Fred_running.

I fully expect the error levels here to be adjusted either during PR review or later during testing. The idea of critical errors, warnings, potential issues, etc was unclear in the old system as it was all sorta jumbled together. The new Error Checker class here tries to unjumble it and make it much clearer to designers what needs to happen with each issue.

I think more can be done here to make this more powerful and clear, but this has grown beyond my original scope already and I think it's in a good enough place to start.

[MjnMixael]

This will need help dialog updates

Done

[wookieejedi]

With the cleaned up distinction between errors and warning all looks good to me now