[5.14.1] UI adjustments

* OS native check of screen resolution (Qt free, experimental)
* changed rounding
* use string for scale factor so we can use fractions as 2/3
* utility functions to clean numbers, parse fractions

It looks like:

* my WIN10 scaling is 250%
* obviously swift rounds that to 3 (device ratio)
* now using FLOOR policy down-rounding to 2
* rational: scaling up (scale factor) is better as down-scaling as the factors would be clearer
* 2->3 1.5, but 3->2 means 0.66667

src/blackgui/guiapplication.h

@@ -284,7 +284,7 @@ namespace BlackGui
 
         //! Support for high DPI screens
         //! \note Needs to be at the beginning of main
-        static void highDpiScreenSupport(double scaleFactor = -1);
+        static void highDpiScreenSupport(const QString &scaleFactor = {});
 
         //! Uses the high DPI support?
         static bool isUsingHighDpiScreenSupport();
@@ -300,8 +300,15 @@ namespace BlackGui
         static void modalWindowToFront();
 
         //! Parse scale factor if any
+        //! \deprecated using scaleFactor now
         static double parseScaleFactor(int argc, char *argv[]);
 
+        //! Get the scale factor
+        static QString scaleFactor(int argc, char *argv[]);
+
+        //! Get a default scale factor
+        static QString defaultScaleFactorString();
+
     signals:
         //! Style sheet changed
         void styleSheetsChanged();