[Bf-committers] progress_update message pumping

[Tom Edwards]

https://codereview.appspot.com/14454056/

This patch makes the wm.progress_update bpy methods pump the message queue.

Results are:

* Users can cancel scripts with Esc
* User input is ignored until the script finishes (except Esc)
* Blender's window can be moved, minimised etc. while the script is running
* On Windows, pumping avoids the "Not Responding" state

Scripts which don't call the progress_update methods (e.g. UI panels) 
are not affected by these changes.

Reviews please!