E.g. when opening the File Browser from the Preferences window, it would be the child of the main window the Preferences belong to. Now it can be a child of the Preferences window itself. The File Browser always stays on top of its parent window. Which avoids some issues with OS window management, see T76418 and T72693. Also removes a now unnecessary workaround to move the child window back to the front after closing the file browser (opening the file browser would move the main window and the file browser to the front). Fixes T76418, T72693.