bzr-colo should have separate shelves per colocated branch.

I would like to accommodate the following workflow:

I work on branch feature1.
Half way through, I decide to switch to colocated branch feature2.
My current uncommitted changes are shelved, and I switch to feature2.
I work on feature2, and half-way through I decide to finish working on feature1.
I switch to feature1, feature2's uncommitted changes are shelved and feature1's previous changes are unshelved.
I work on feature1, finish and commit.
I switch to feature2, the uncommitted changes are unshelved into my working directory.

Basically, something like automatic shelving/unshelving when switching, or one shelf per branch.

I've posted this here in case it's more relevant, it was originally on:

https://bugs.launchpad.net/bzr/+bug/731729