Provide more updates as things are executed

[cryos]

The Tomviz interface is far too quiet, we should work to provide timely updates in the status bar, and probably in the application log. Especially for large data sets it can be disorienting. I think for a typical external pipeline run we might offer:

1. Writing data to input file at ...
2. Executing pipeline (already there)
3. Reading transformed data from ...
4. ...

It would be good to look at putting reads and writes in a background thread too.