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.

docs/changes.txt

@@ -128,6 +128,7 @@ All (GUI):
 - Add wxGrid::SetCornerLabelValue() (Pavel Kalugin).
 - Add strikethrough support for fonts defined in XRC.
 - Add wxDisplay::GetPPI().
+- Add wxGraphicsContext::GetWindow().
 
 wxGTK: