Tests, introduce ResourceSet interface

[generateui]

• add SOCResourceSet tests
• introduce ResourceSet interface
• move fixed resourcesets to their owner class (city, road, ship, settlement)
• improve .gitignore
• tidy up

[generateui]

A ResourceSet interface introduces simplicity, safety, clarity, testability and readability.

• safety: We'll be sure the menthods will not actually accidentally modify the resource set when they're not supposed to.
• clarity: We're communicating "here is some info, but you are not supposed to alter any data here".
• simplicity: methods/classes should depend on as less as possible. An immutable ResourceSet exposes a lot less stuff then the mutable SOCResourceSet.
• testability: Since it is an interface, method call can get mock/fake implementations, so it's suddenly possible to test
• readbility: see reasons of simplicity and clarity

Above arguments can be used for any type in the game (why not have ImmutablePlayer, ImmutableGame, etc... ?). ResourceSet deserves it's own since it is used a lot throughout the codebase. It's a very common concept used to implement things in JSettlers. For other classes, this advantage is (much) less so, so introducing an immutable interface definition for it is probably overkill.

[jdmonin]

That makes sense, thanks again. Merged, with followups as noted in review.