Graphviz layout does nothing in 0.10

[dan-van]

This is a regression of
#177
#295

Steps to reproduce:
Start Gephi 0.10.
Generate a random graph.
Use Graphviz layout plugin to change layout.

Nothing happens. No logging, no change in layout.

However, this bug has been fixed in this pull request: #321
(but also see this proposed addition to that pull request to add more handling, logging, and testing: dan-van@e990347)