GTK: window-height doesn't take tab bar into account

#2660 but GTK. This is a documented problem in Config.zig but somehow we never got around to filing an issue for this.

For a long time we thought this was impossible since GTK 4 does not allow you to set the default size of a widget directly, but you can do it with a bit of elbow grease and gtk.WidgetClass.measure. I have a fix at https://github.com/pluiedev/ghostty/commit/ebea7ccc95885f5174d8d59ef90e69e94cbf4dee which depends on #14052.