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.
This change allows semantic-sticky-func-mode to display parameters that
are scattered in many lines by using stickyfunc-enhance package. It also
specifically handles Python to also display assigned values if
available.
Also update documentation.
