Windows without their own menu bar can inherit an incorrect one

[GriffinRichards]

Example:

• Launch Porymap and open the Shortcuts Editor (Options -> Shortcuts...).
• Open the Tileset Editor window (Tools -> Tileset Editor)
• Select the Shortcuts Editor window. The Tileset Editor menu bar is retained.
• Select and close the Tileset Editor. Focus returns to the Shortcuts Editor window, but now it retains the Tileset Editor menu bar even after reselecting the main editor window.

Some parts of the Qt docs suggest (for macOS, at least) that there should only ever be one menu bar, so we might be doing this wrong with the Tileset Editor