Tiled should warn when loading Text Objects with missing fonts #3548

When opening maps that contain Text Objects that refer to a font that Tiled cannot find, Tiled has good behaviour: it uses a default placeholder font, and keeps the reference. However, the user is not informed why this is happening, and this can be confusing when the user thinks they have the font (e.g. if they have "MyFont Regular" instead of "MyFont Light").

I think when opening Maps and Templates with Text Objects with missing fonts, Tiled should show a warning letting the user know that the font could not be found, and this warning should include the exact name of the font as it appears in the Object. If there's room, it might be good to also mention that the font reference will be saved as-is or that a placeholder font will be displayed, to alleviate fears of data loss.