Enhance proof project nature: add proof project preference, use prover-specifics

[David Aspinall]

The nature attached to proof projects can be used to set the prover and then some preferences on the prover for working within that project. This allows us to have project-specific prover settings, such as:

• symbol table: this can be the copied default file as before, or the standard default (to save copying)
• prover command: again, copied default or project-specific override

Also, perhaps we could use the project nature to influence the "new proof script action" so that this automatically determines the prover from the container project (not relevant for the menu action which doesn't select the container beforehand, though).

[David Aspinall]

Milestone PG-Eclipse-1.1.0 deleted