#26318 global engine progress overlay

Adds **internal** functionality to pop a progress UI to indicate that a long running activity is happening:

![screen shot 2014-09-02 at 15 37 32](https://cloud.githubusercontent.com/assets/337710/4119980/122add82-32af-11e4-8f5e-618115a8eabc.png)

The intent is to use this for the path cache sync scenarios which may take longer than usual, for example a full sync. The progress only pops up on engines which have UI (QT) capabilities - other engines will log an info message instead.
this is going to be modeless, right?  The comment at the top says it will be modal...  did you intend to use window.exec_()?