Move company and auto-complete to a common layer.
They are not enabled globally anymore, each mode using them
must explicitly declare a hook.
Only one frontend is supported for a given mode, we have to
choose the best between the two.
Only one key binding to toggle auto-completion on `SPC t a` no
matter if it is company or auto-complete. The lighter in the
mode-line is Ⓐ for both frontends.
- Enable lazy-loading of company.
- The company layer is now agnostic of any packages which use company
- company-backends is now buffer local and contains only the backends
which are relevant for the current modes
- New macro `spacemacs|reset-local-company-backends`
