It is important to define the buffer local backend variables
in a config.el otherwise it is not available in the layer
configuration if the auto-completion layer is removed.
There were also some issues by hiding the explicit
configuration of company inside a macro, for instance
helm-spacemacs is unable to display the company package
configured in a given layer if the auto-completion layer
is removed.
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.
