Add wxWindow::GetDPI()

This is simpler to use than wxDisplay(window).GetPPI() which was used
instead of it so far in all ports and can be implemented more
efficiently for wxMSW.

Remove wxGetWinTLW, GetDPI already tries to get the top window.

include/wx/window.h

@@ -949,14 +949,16 @@ public:
     // translation between different units
     // -----------------------------------
 
+        // Get the DPI used by the given window or wxSize(0, 0) if unknown.
+    virtual wxSize GetDPI() const;
+
         // DPI-independent pixels, or DIPs, are pixel values for the standard
         // 96 DPI display, they are scaled to take the current resolution into
         // account (i.e. multiplied by the same factor as returned by
         // GetContentScaleFactor()) if necessary for the current platform.
         //
-        // Currently the conversion factor is the same for all windows but this
-        // will change with the monitor-specific resolution support in the
-        // future, so prefer using the non-static member functions.
+        // To support monitor-specific resolutions, prefer using the non-static
+        // member functions or use a valid (non-null) window pointer.
         //
         // Similarly, currently in practice the factor is the same in both
         // horizontal and vertical directions, but this could, in principle,