Explicitly skip expanding stretchable separators in MSW toolbars

When computing the fixed size of MSW toolbars, don't take stretchable
separators into account as this doesn't make sense and so, even if this
doesn't matter currently because the separators still have their
initial, fixed size when total fixed size is computed, but would start
mattering if UpdateStretchableSpacersSize() is ever called before this
is done and it also just makes more sense to skip them here.

src/msw/toolbar.cpp

@@ -1200,10 +1200,15 @@ bool wxToolBar::Realize()
 
         if ( !tool->IsControl() )
         {
-            if ( IsVertical() )
-                m_totalFixedSize += r.bottom - r.top;
-            else
-                m_totalFixedSize += r.right - r.left;
+            // Stretchable space don't have any fixed size and their current
+            // size shouldn't count at all.
+            if ( !tool->IsStretchableSpace() )
+            {
+                if ( IsVertical() )
+                    m_totalFixedSize += r.bottom - r.top;
+                else
+                    m_totalFixedSize += r.right - r.left;
+            }
 
             continue;
         }