WIP: Allow staging multiple elements in `bst shell`

Description

This makes changes to internal APIs, ABI and the CLI to make it possible to stage multiple elements.

Defining a pseudo-element that depended on all the elements on the CLI was considered over changing internal APIs, but the API surface of Element was too large to make a convincing mock and would have been at least as much effort.

Changes proposed in this merge request:

• Add BST_GRANULAR_STAGE flag for Elements to set if they support staging dependencies, running integration commands and other staging operations individually rather than all at once in Element.stage().
• Port staging logic to prefer to call multiple staging methods if BST_GRANULAR_STAGE is set.
• Port core Elements to support BST_GRANULAR_STAGE.
• Move shell set-up code out of Element into Stream.
• Make Stream.shell support multiple elements
• Add -e flag to bst shell CLI to specify multiple elements.

This merge request, when approved, will close issue/bug: #422

CHANGELOG/Release Notes

• Initial WIP version, some comments and checking for having been passed no elements are yet to be addressed.
• Element checking, resolve WIP comments and update NEWS.
• Fixed a rebasing error and lint warnings.