Zone importer dialog

[Moneyl]

Implement a modal popup for zone import.

• Show when user tries to open a map that hasn't been opened in their current project.
• Add progress bar
• Lock off the rest of the UI until import finishes. Want to prevent user accidentally breaking something due to multiple threads interacting. To start the rewrite is mostly gonna be single threaded. During import the render/UI thread and importer thread should be mostly independent.
• Let users pick multiple maps to import and put them in a queue. They'll be imported one by one.
• Change the main menu bar to only show imported maps or somehow indicate which ones have been imported so far. If the former is chosen have a separate menu option to open the importer dialog.