Make possible to copy more stuff

- local branch name
- remote branch name
- untracked files
- (un)staged files
magit-insert-status-{local,remote}-line: use `branch' as section type

Adjust `magit-copy-item-as-kill' accordingly.
magit-copy-item-as-kill: cosmetics
Merge branch 'improve--magit-copy-item-as-kill'
Merge branch 'master' into next