Add wxVector::shrink_to_fit() for C++11 compatibility

Also use this for wxArray::Shrink() implementation as it's more
efficient than the old swap-based implementation which requires an extra
memory allocation instead of really shrinking the existing one.

interface/wx/vector.h

@@ -262,6 +262,18 @@ public:
     void resize(size_type n, const value_type& v);
     //@}
 
+    /**
+        Free unused memory allocated by the vector.
+
+        Reduces the memory used by the vector to the bare minimum required to
+        hold its current number of elements, possibly 0.
+
+        After calling this method, capacity() returns the same as size().
+
+        @since 3.1.1
+     */
+    void shrink_to_fit();
+
     /**
         Returns the size of the vector.
     */