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>

examples/scrollingdetector.py

@@ -0,0 +1,58 @@
+import os
+import time
+
+from gi.repository import Gtk
+
+from sugar3.graphics import style
+from sugar3.graphics.icon import CellRendererIcon
+from sugar3.graphics.xocolor import XoColor
+from sugar3.graphics.scrollingdetector import ScrollingDetector
+import common
+
+
+test = common.Test()
+test.show()
+
+model = Gtk.ListStore(str)
+
+data_dir = os.getenv('GTK_DATA_PREFIX', '/usr/')
+
+iconlist = os.listdir(os.path.join(data_dir,
+                      'share/icons/sugar/scalable/actions/'))
+print "Displaying %s icons" % len(iconlist)
+for icon in iconlist:
+    icon = os.path.basename(icon)
+    icon = icon[:icon.find('.')]
+    model.append([icon])
+
+scrolled = Gtk.ScrolledWindow()
+scrolled.set_size_request(800, 800)
+treeview = Gtk.TreeView()
+
+treeview.set_model(model)
+scrolled.add(treeview)
+test.pack_start(scrolled, True, True, 0)
+test.show_all()
+
+col = Gtk.TreeViewColumn()
+treeview.append_column(col)
+
+xo_color = XoColor('#FF0000,#00FF00')
+cell_icon = CellRendererIcon(treeview)
+cell_icon.props.width = style.GRID_CELL_SIZE
+cell_icon.props.height = style.GRID_CELL_SIZE
+cell_icon.props.size = style.STANDARD_ICON_SIZE
+cell_icon.props.xo_color = xo_color
+
+col.pack_start(cell_icon, expand=False)
+col.add_attribute(cell_icon, 'icon-name', 0)
+cell_text = Gtk.CellRendererText()
+col.pack_start(cell_text, expand=True)
+col.add_attribute(cell_text, 'text', 0)
+
+detector = ScrollingDetector(scrolled)
+detector.connect_treeview(treeview)
+
+if __name__ == '__main__':
+    time_ini = time.time()
+    common.main(test)