Add wxGraphicsContext::GetWindow()

This method allows to retrieve the window this context is associated
with, if any.

Add "wxWindow*" argument to wxGraphicsContext ctor and provide the
window pointer to it when available, i.e. when creating the context from
a wxWindow directly or from wxWindowDC, which is also associated with a
window, in platform-specific code.

No real changes yet.

interface/wx/graphics.h

@@ -1095,6 +1095,19 @@ public:
     */
     virtual void GetDPI( wxDouble* dpiX, wxDouble* dpiY);
 
+    /**
+        Returns the associated window if any.
+
+        If this context was created using Create() overload taking wxWindow or
+        wxWindowDC, this method returns the corresponding window. Otherwise
+        returns @NULL.
+
+        @return A possibly @NULL window pointer.
+
+        @since 3.1.2
+     */
+    wxWindow* GetWindow() const;
+
     /** @}
     */