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.

src/common/graphcmn.cpp

@@ -565,12 +565,14 @@ void * wxGraphicsBitmap::GetNativeBitmap() const
 wxIMPLEMENT_ABSTRACT_CLASS(wxGraphicsContext, wxObject);
 
 
-wxGraphicsContext::wxGraphicsContext(wxGraphicsRenderer* renderer) :
-    wxGraphicsObject(renderer),
+wxGraphicsContext::wxGraphicsContext(wxGraphicsRenderer* renderer,
+                                     wxWindow* window)
+    : wxGraphicsObject(renderer),
       m_antialias(wxANTIALIAS_DEFAULT),
       m_composition(wxCOMPOSITION_OVER),
       m_interpolation(wxINTERPOLATION_DEFAULT),
-      m_enableOffset(false)
+      m_enableOffset(false),
+      m_window(window)
 {
 }