[GUI]: Refactor control of long-running computational processes

[tturocy]

Currently, the graphical interface makes calls to the external gambit-* binaries to compute equilibria.

We would like to accomplish the following:

• Refactor this to be more robust - we will want to continue to have this as a possibility as sometimes we must run external tools
• Develop a version of the controller that runs in a thread - this would be the preferred way to support "Gambit-native" implementations to avoid complications with finding the locations of tools, and so on.