Window management issue with the fonts dialog on Mac

[lmoureaux]

Describe the bug

https://osdn.net/projects/freeciv/ticket/44851

Confirmed by dino in beta1

[lmoureaux]

Strange, the color picker is parented correctly on Linux. Might be a limitation of Qt or the window manager on Mac OS (dialog in dialog).

[lmoureaux]

From the docs for QDialog:

Note: The parent relationship of the dialog does not imply that the dialog will always be stacked on top of the parent window. To ensure that the dialog is always on top, make the dialog modal. This also applies for child windows of the dialog itself. To ensure that child windows of the dialog stay on top of the dialog, make the child windows modal as well.