Initialize wxTopLevelWindow::DecorSize in wxGTK

Add default ctor for this struct as it was too easy to forget to
initialize it otherwise, ending up with bogus values in it, as it
happened with wxTLWGeometry::m_decorSize, which resulted in a failure in
wxPersistTLW unit test and, probably, real code too.

src/gtk/toplevel.cpp

@@ -565,7 +565,6 @@ void wxTopLevelWindowGTK::Init()
     m_updateDecorSize = true;
     m_netFrameExtentsTimerId = 0;
     m_incWidth = m_incHeight = 0;
-    memset(&m_decorSize, 0, sizeof(m_decorSize));
 
     m_urgency_hint = -2;
 }