spacemacs/add-flycheck-hook was not really hooking anything, change the name
to better reflect what it does.
Also changed the push for a add-to-list to avoid duplicates.
Enabling a company backend for a specific mode was a tedious tasks with code
scattered at different locations, one for local variable definitions, one for
company hook function definitions and another where the backends were pushed to
the local variables (which was problematic, since we ended up pushing the same
backends over and over again with `SPC f e R`, pushes have been replaced by
add-to-list calls in the new macro).
All these steps are now put together at one place with the new macro
spacemacs|add-company-backends, check its docstring for more info on its
arguments.
This macro also allows to define arbitrary buffer local variables to tune
company for specific modes (similar to layer variables via a keyword :variables)
The code related to company backends management has been moved to the
auto-completion layer in the funcs.el file. A nice side effect of this move is
that it enforces correct encapsulation of company backends related code. We can
now easily detect if there is some configuration leakage when the
auto-completion layer is not used. But we loose macro expansion at file loading
time (not sue it is a big concern though).
The function spacemacs|enable-auto-complete was never used so it has been
deleted which led to the deletion of the now empty file core-auto-completion.el.
The example in LAYERS.org regarding auto-completion is now out of date and has
been deleted. An example to setup auto-completion is provided in the README.org
file of the auto-completion layer.
