qtFRED variables editor
Draft because there are a couple more issues to iron out with character limits of list and map items and applying the changes to the mission, but it's close enough to being done that people can start taking a look. The basic functionality is pretty well tested, but I think we'll need some real-world testing once it's in.
Note: This allows duplicate keys until the dialog is accepted, at which point it checks for duplicate keys, warns the user and prevents saving, listing the exact items that are problematic. This allows users to make edits without qtFRED erasing user inputs if they happen to be duplicates. But that means that keys are not really keys until saved to the mission.
There's some new features as well, like value-key switching. Variable and container copying, etc.
While I still have to do a last run of tests on the last two features I added, this is now basically done and ready for review.
Double checking that this is ready for review?
Yesum
Note this fixes #3818