This adds the titular feature, as well as FRED support. There are also some associated minor UI fixes in FRED and some cleanup in missiongoals.cpp. And "Event Flags" are now a thing in case we want to add any other flags in the future.
Not resizing the Messages vector here was leading to a debug error about an out-of-range vector subscript, and who knows what other problems were being caused by overwriting whatever memory was after the Messages vector. There's no equivalent problem when parsing, because as each message is parsed, it gets pushed onto the back of the vector instead of assigned directly to an index.
Goober5000