Add wxNumberFormatter::Format() for floating point numbers

The existing ToString() is not flexible enough to be used in wxGrid,
which supports specifying the width (and not just the precision) as well
as using formats other than "%g" and "%f" which are the only ones
supported by the existing function.

Note that currently the implementation simply calls wxString::Format()
and then adjusts the decimal separator, but it could, in principle, use
wxUILocale methods for formatting the floating point numbers using
native platform functions for doing this, e.g. CFNumberFormatter under
macOS.

interface/wx/numformatter.h

@@ -93,6 +93,16 @@ public:
     static wxString
     ToString(double val, int precision, int flags = Style_WithThousandsSep);
 
+    /**
+        Format the given number using one of the floating point formats and
+        ensure that the result uses the correct decimal separator.
+
+        Prefer using ToString() if possible, i.e. if format is "%g" or "%.Nf"
+        which are supported by it directly.
+
+        @since 3.1.6
+     */
+    static wxString Format(const wxString& format, double val);
 
     /**
         Parse a string representation of a number possibly including thousands