Support multiple installed engines

[CharliePoole]

This may end up being a non-issue, if our existing code takes care of the job, but I have a suspicion it doesn't. Here's what we need to verify and/or implement:

• Multiple engines can be present in well-known locations or pointed to from the registry or settings.
• All such engines should be examined by the EngineActivator when deciding which engine to use.
• Each engine may potentially have it's own set of extensions.
• Each engine may also point to a set of shared extensions.
• In addition to such "findable" engines, any runner may be packaged with a private engine. Such an engine will not be available to other runners.
• The engine that is found and used by a runner should be easily identifiable by the user, in order to facilitate bug reporting and debugging.

While this is not a user-facing feature, being able do the above is a prerequisite for other features we want to implement.

[CharliePoole]

Postponing this... it's important but not urgent.

[CharliePoole]

Issue moved to nunit/nunit-console #3 via ZenHub