This change will make buffers with the motion state
(e.g., spacemacs home buffer) be able to use the
evil-unimpaired navigation key bindings.
Note that the normal state will inherit the motion
state map anyway, so there's no need to set the
normal state map separately.
Prevent next-buffer, other-buffer, etc. from choosing useless buffers.
No need for spacemacs/next-useful-buffer,
spacemacs/previous-useful-buffer anymore.
Also fix spacemacs/alternate-buffer to respect buffer-predicate.
When spacemacs-layouts is used, buffer-predicate filters useful buffer
that belong to the current layout.
