gitk: Implement a simple scheduler for the compute-intensive stuff

This allows us to do compute-intensive processing, such as laying out
the graph, relatively efficiently while also having the GUI be
reasonably responsive.  The problem previously was that file events
were serviced before X events, so reading from another process which
supplies data quickly (hi git rev-list :) could mean that X events
didn't get processed for a long time.

With this, gitk finishes laying out the graph slightly sooner and
still responds to the GUI while doing so.

Signed-off-by: Paul Mackerras <paulus@samba.org>