Move scale factor-related functions to wxGDIImage from wxBitmap

This makes them available in wxIcon (and, less importantly, wxCursor)
too which is needed in order to use icons correctly in high DPI.

This is also more consistent with the other platforms, where wxIcon does
have these methods.

Document these methods in wxIcon now that they're available there under
all platforms and also document wxIcon::GetSize() which had been
available even before, but wasn't documented.

include/wx/msw/bitmap.h

@@ -191,19 +191,6 @@ public:
     void UseAlpha(bool use = true);
     void ResetAlpha() { UseAlpha(false); }
 
-    // allow setting and storing the scale factor
-    virtual void SetScaleFactor(double scale);
-    virtual double GetScaleFactor() const;
-
-    // return the size divided by scale factor
-    wxSize GetDIPSize() const;
-
-    // logical metrics accessors return the same thing as physical ones, just
-    // as in all the other ports without wxHAS_DPI_INDEPENDENT_PIXELS.
-    double GetLogicalWidth() const;
-    double GetLogicalHeight() const;
-    wxSize GetLogicalSize() const;
-
     // old synonyms for CreateWithLogicalSize() and GetLogicalXXX() functions
     bool CreateScaled(int w, int h, int d, double logicalScale)
         { return CreateWithLogicalSize(wxSize(w, h), logicalScale, d); }