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/generic/graphicc.cpp

@@ -1830,7 +1830,7 @@ wxCairoContext::wxCairoContext( wxGraphicsRenderer* renderer, const wxPrinterDC&
 #endif
 
 wxCairoContext::wxCairoContext( wxGraphicsRenderer* renderer, const wxWindowDC& dc )
-: wxGraphicsContext(renderer)
+: wxGraphicsContext(renderer, dc.GetWindow())
 {
     int width, height;
     dc.GetSize( &width, &height );
@@ -2244,7 +2244,7 @@ wxCairoContext::wxCairoContext( wxGraphicsRenderer* renderer, cairo_t *context )
 }
 
 wxCairoContext::wxCairoContext( wxGraphicsRenderer* renderer, wxWindow *window)
-    : wxGraphicsContext(renderer)
+    : wxGraphicsContext(renderer, window)
 #ifdef __WXMSW__
     , m_mswWindowHDC(GetHwndOf(window))
 #endif