Make multi-buffer support useful for regular file editing

[ardabro]

Body

What are you proposing?

Multi-buffers have great potential, but currently, they are not very useful for regular multi-file editing.
I'd like to be able to gather several related views into a single multibuffer for easier introspection, navigation, and source code editing. The main limitation is the lack of at least two crucial features:

1. Adding (and removing) a view to/from an existing multibuffer.
2. Resizing (shrinking) an open view.

Why does this matter?

Because it would bring multibuffers to a new level of usefulness.
And until a bookmarks feature is available, this would be even more valuable.