First step towards multiple working copies

[guillep]

The current repository-centric design allows to have only single working copy. Indeed, before the repository used to do

Repository >> foo
   self workingCopy foo

This PR moves several responsibilities from the repository to the working copy object. A lot of code gets clearer, allows to have multiple working copies, and as backwards compatibility the repository still has a default working copy as before.

Other changes:

• IcePackage knows the working copy and not the repository (this is better because a package may exist in a commit and not on another)
• Some UI models got updated to work on the working directory
• I renamed/aliased some very generic methods that made coding very difficult (model, entity => repositoryModel, workingCopyModel, repository, workingCopy). The generic names imply a false polymorphism. There is no polymorphic usage between a workingCopy and a repository except for asking for a description. The uses I stumbled upon use now the less generic version that correspond on each situation.
• removed IceTipCache et al that complicated debugging and modifications for not much

[Ducasse]

@guillep there are lot of tests of iceberg that broke.

[Ducasse]

Do you want that I give a try to get some corner cases just using the UI?

[guillep]

If you want, but just FYI, I redid all these changes from scratch so this is much cleaner than what we drafted yesterday. I'm much more confident on this PR than on what we did yesterday ^^.

[Ducasse]

Ok I do not have time right now so once this is integrated I will use it and we will see :)