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/listctrl.h

@@ -380,6 +380,8 @@ protected:
     // common part of all ctors
     void Init();
 
+    virtual bool MSWShouldSetDefaultFont() const wxOVERRIDE { return false; }
+
     // Implement constrained best size calculation.
     virtual int DoGetBestClientHeight(int width) const wxOVERRIDE
         { return MSWGetBestViewRect(width, -1).y; }