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.
Previously,
* typing `gf` in domain names (for example google.com) in normal buffers
* typing `<tab>` in domain names in the minibuffer
would start pinging that domain name. This is not really useful, so
disable it.
Fixes#2654.
Helm-flx, which is included as a core package, requires a minimum Emacs
version of 24.4. As it stands attempting to install Spacemacs on Emacs
24.3 or below will break on helm-flx.
