[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/blackgui.pro

@@ -25,6 +25,9 @@ else {
     QMAKE_CXXFLAGS += -idirafter $$EXTERNALSROOT/common/include/qwt
 }
 
+# needed for "ShellScalingApi.h" only
+# win32 { LIBS *= -lSHCore }
+
 DEPENDPATH += . ..
 
 PRECOMPILED_HEADER = pch/pch.h