Ability to manually optimize build graph

This is mostly a request for comments at this point. There's some exceptional build items (read: take very long to compile) in freedesktop-sdk like LLVM that are the reverse dependencies of many things and as a consequence you'd want to to start compiling them as early as possible. In other words, not all elements are really equal.

Should BuildStream expose some tools (eg element priority) to projects to be able to have more control over element build order? Is it already in current element model possible to affect that without adding artificial dependencies?