Fix and refactor editor puzzle import

Fixes #227 (closed)

Previously:

• Importing a puzzle from the greeter was broken. The greeter used a PuzzleDownloader to open a file dialog, but it didn't register a listener for the finished signal. So nothing happened after the user selected a file.
• Importing from the edit window worked.
  • The edit window handled the importing on its own, instead of relying on a PuzzleDownloader.
  • It only supported IPUZ files, as opposed to PuzzleDownloader, which supports other file types.
  • It didn't open the new puzzle in a new window.

For this MR:

• I created an edit_window_start_with_file_explorer function. This function does the following:
  1. Present the user with a file explorer.
  2. Start a new edit window using the puzzle selected by the user.
  3. Close the previous window (either a greeter or an edit window).
• Both the greeter and edit window now use this function to handle puzzle imports.