scripts/pybootchart: Port to python3

scripts/pybootchart: Port to python3

This updates the pybootchart code (used for viewing build timing profiles)
to use python3. The bulk of the changes are to use gi instead of pygtk, i.e.
port from gtk+2 to gtk+3.

The main change is to make the bootchart widget inherit gtk.Scrollable
and change the way the scrollbars are implemented to match the new method
upstream. The drawing code used cairo already so can remain unchanged,

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>