Avoid changing checkbox background in wxGrid under non-MSW

Doing this makes the checkbox unusable with the default GTK 3 theme as
the default grid background colour (white) is the same as the colour of
the check mark -- so changing the checkbox background to it makes it
invisible.

Work around this by adding a new SetTransparentPartColour() method that
can be used by wxGrid (and, in the future, user code if we decide that
this is really the best solution to this problem that we can provide) to
make the checkbox blend in with its background without actually changing
its appearance.

include/wx/msw/checkbox.h

@@ -45,6 +45,11 @@ public:
     // override some base class virtuals
     virtual void SetLabel(const wxString& label) wxOVERRIDE;
 
+    virtual void SetTransparentPartColour(const wxColour& col) wxOVERRIDE
+    {
+        SetBackgroundColour(col);
+    }
+
     virtual bool MSWCommand(WXUINT param, WXWORD id) wxOVERRIDE;
     virtual void Command(wxCommandEvent& event) wxOVERRIDE;