Fix Bower update getting stuck + Bower install/updage not showing "Getting packages..."

@keertip

#1876 didn't completely upgrade Bower manager along the same lines as Pub one. It was thinking it was operating on a `Project`, while in fact it was a `Folder`. That caused an exception in 'Bower update', making it appear stuck.

I believe that somehow prevented the "Getting packages..." progress message from being displayed for Bower commands.

This CL:

* Moves both package managers, their abstract parent, and related commands in `Spark` to operate in terms of `Folder`, from a mix of `Container` (new after #1876) and `Project` (vestiges from before): `Folder` is what they will always get by design (`Project` is a subclass of `Folder`).
* Fixes #2263, which I've just filed for documentation purposes.
* Fixes #2118 and #2257