Replace dynamic casts with MSWShouldSetDefaultFont() virtual method

Get rid of ugly wxDynamicCastThis()s in wxMSW wxControl code and add a
new virtual method overridden in wx{Tree,List}Ctrl instead.

Also stop comparing the font with wxSYS_DEFAULT_GUI_FONT, there doesn't
seem to be any good reason to set this particular font for these
controls neither.

In addition to simplifying and de-ugligying the code, this commit
incidentally fixes -Wnonnull-compare warnings in this code from gcc6
too.

include/wx/msw/treectrl.h

@@ -211,6 +211,8 @@ protected:
                            int width, int height,
                            int sizeFlags = wxSIZE_AUTO) wxOVERRIDE;
 
+    virtual bool MSWShouldSetDefaultFont() const wxOVERRIDE { return false; }
+
     // SetImageList helper
     void SetAnyImageList(wxImageList *imageList, int which);