Do not switch to a abacus already selected

This change avoid one redraw at startup in the default case
Fix import in abacus.py
Add a methods to allow delay of draw

Until now, every Sprite request the draw of their own region.
This patch transfer that responsability to the Sprites object,
adding methods to delay the drawing to allow initialize all the sprites
without doing any draw.
Now, the aplication need call the Sprites.init() method,to allow the drawing.
This is needed to optimize the startup time, requesting only one draw.

Signed-off-by: Gonzalo Odiard <godiard@sugarlabs.org>
Draw only one time on activity startup

abacus.py application is also modified
Draw sprites using cairo surfaces

The objectives is see if the performance can be improved
Remove unneded imports