Restore support for span

Blitting is not used in matplotlib GTK3Cairo backend, disabled here as well.
SpanSelector and RectangleSelector now have same properties "active" and "visible" (see https://github.com/matplotlib/matplotlib/pull/3376 )
FIXME: On layout change, more than one SpanSelector appears on Graph
Use RectangleSelector from matplotlib

Since matplotlib PR #619 was merged