Implement C(-S)-tab buffer switch in Helm layer

Equivalent to PR syl20bnr#14287 for ivy-layer

CHANGELOG.develop

@@ -2247,6 +2247,7 @@ Other:
   - Add ~C-d~ to create a project perspective and open a project root dired
     buffer, from the ~SPC p l~ persp switch project prompt
     (thanks to Magnus Therning)
+  - Add =C(-S)-tab= keybindings for buffer switching (thanks to Daniel Nicolai)
 - Fixes:
   - Fixed signature of =spacemacs//display-helm-window= (thanks to Jack Kamm)
   - Fixed initialization by calling =helm-mode= when Helm is initialized=

doc/DOCUMENTATION.org

@@ -2440,11 +2440,12 @@ The mode can be toggled on and off with ~SPC t g~.
 
 *** Buffers and Files
 **** Buffers manipulation key bindings
-Buffer manipulation commands (start with ~b~):
+With few exceptions, buffer manipulation commands (start with ~b~):
 
 | Key binding     | Description                                                                            |
 |-----------------+----------------------------------------------------------------------------------------|
 | ~SPC TAB~       | switch to alternate buffer in the current window (switch back and forth)               |
+| ~C-TAB/C-S tab~ | cycle backward/forward through previous visited buffers (i.e. scroll helm buffer list) |
 | ~SPC b b~       | switch to a buffer                                                                     |
 | ~SPC b d~       | kill the current buffer (does not delete the visited file)                             |
 | ~SPC u SPC b d~ | kill the current buffer and window (does not delete the visited file)                  |

layers/+completion/helm/packages.el

@@ -85,6 +85,9 @@
         (global-set-key (kbd "M-x") 'spacemacs/helm-M-x-fuzzy-matching))
       (global-set-key (kbd "C-x C-f") 'spacemacs/helm-find-files)
       (global-set-key (kbd "C-x b") 'helm-buffers-list)
+      ;; use helm to switch last(/previous) visited buffers with C(-S)-tab
+      (evil-global-set-key 'motion (kbd "<C-tab>") 'helm-buffers-list)
+      (evil-global-set-key 'motion (kbd "<C-iso-lefttab>") 'helm-buffers-list)
       ;; use helm everywhere
       (spacemacs||set-helm-key "<f1>" helm-apropos)
       (spacemacs||set-helm-key "a'"   helm-available-repls)
@@ -169,6 +172,9 @@
       (setq helm-white-buffer-regexp-list
             (append helm-white-buffer-regexp-list
                     spacemacs-useful-buffers-regexp))
+      ;; use helm to switch last(/previous) visited buffers with C(-S)-tab
+      (define-key helm-map (kbd "<C-tab>") 'helm-follow-action-forward)
+      (define-key helm-map (kbd "<C-iso-lefttab>") 'helm-follow-action-backward)
       ;; alter helm-bookmark key bindings to be simpler
       (defun simpler-helm-bookmark-keybindings ()
         (define-key helm-bookmark-map (kbd "C-d") 'helm-bookmark-run-delete)