Data folder selector + validation

[Moneyl]

Create a modal dialog for editing and validating the RFG data folder.

• [x] Popup automatically if the data folder is not set or validation fails.
• [x] List of validation errors/warnings. - [ ] Attempt to detect when incompatible versions of RFG like steam edition or glfw edition are selected.
• [x] Warn if critical vpps like misc, mp_common, or zonescript_terr01 are missing. Decent sign that they picked the wrong folder.
• [x] Add Select data folder... option to main menu bar.

[Moneyl]

Completed by ae89a14639af15798ce19a91a068a18722ca6460