Detect scrolling in treviews and optimize drawing

Detecting if the treeview is scrolling we can improve the performance of
the CellRendererIcon by avoiding unneeded calculations.

The example scrollingdetector.py shows how this can be used.  The
changes are backward compatible.

Signed-off-by: Manuel Quiñones <manuel.por.aca@gmail.com>
Signed-off-by: Gonzalo Odiard <godiard@sugarlabs.org>

src/sugar3/graphics/Makefile.am

@@ -18,6 +18,7 @@ sugar_PYTHON =              \
 	panel.py                \
 	radiopalette.py         \
 	radiotoolbutton.py      \
+	scrollingdetector.py    \
 	style.py                \
 	toggletoolbutton.py     \
 	toolbarbox.py           \