Write documentation for sugar3.graphics.ToolButton

- add two examples,

- document critical properties as keyword arguments in the class init,

- ensure documentation is consistent between RadioToolButton and
  ToolButton, (as some of the text is similar),

- part of feature Sugar3 Docs,
  https://wiki.sugarlabs.org/go/Features/Sugar3_Docs

- follow How To Guide
  https://wiki.sugarlabs.org/go/Features/Sugar3_Docs/How_To_Write

Derived from work by

- Hrishi <hrishipatel99@gmail.com> (31176ea) and

- radicalonion <ndefilippis98@gmail.com> (db89aca).

examples/toolbutton.py

@@ -0,0 +1,35 @@
+from gi.repository import Gtk
+
+from sugar3.graphics.toolbarbox import ToolbarBox
+from sugar3.graphics.toolbutton import ToolButton
+
+import common
+
+
+test = common.Test()
+
+vbox = Gtk.VBox()
+test.pack_start(vbox, True, True, 0)
+
+toolbar_box = ToolbarBox()
+vbox.pack_start(toolbar_box, False, False, 0)
+
+separator = Gtk.SeparatorToolItem()
+toolbar_box.toolbar.insert(separator, -1)
+
+
+def __clicked_cb(button):
+    n = int(button.get_tooltip())
+    button.set_tooltip(str(n+1))
+    print "tool button click count %d" % n
+
+tool_button = ToolButton(icon_name='view-radial', tooltip='0')
+tool_button.connect('clicked', __clicked_cb)
+tool_button.set_hide_tooltip_on_click(False)
+tool_button.set_accelerator('<Space>')
+toolbar_box.toolbar.insert(tool_button, -1)
+
+test.show_all()
+
+if __name__ == '__main__':
+    common.main(test)