FlightDisplayViewWidgets: Scale instrument size with the configured font size

On large/high DPI screens, the whole UI scales nicely when increasing the
font size, except for the attitude widget, that was limited to 200 pixels.

src/FlightDisplay/FlightDisplayViewWidgets.qml

@@ -42,8 +42,7 @@ Item {
         if(ScreenTools.isMobile) {
             return ScreenTools.isTinyScreen ? mainWindow.width * 0.2 : mainWindow.width * 0.15
         }
-        var w = mainWindow.width * 0.15
-        return Math.min(w, 200)
+        return ScreenTools.defaultFontPixelWidth * 30
     }
 
     function _setInstrumentWidget() {