refactor: doom-switch-frame-hook

Redesign this hook around `after-focus-change-function`, along with
debouncing, to prevent it from triggering too aggressively (due to
misbehaving desktop environments, elisp packages that tinker with frame
focus, or accidental (and rapid) focus changes by the user).

`doom-switch-{window,buffer}-hook` have also been simplified, and
`doom-switch-window-hook` now will not trigger when focusing another
frame (only when you switch windows *within* any one frame).

This also fixes diff-hl not updating when refocusing an Emacs frame.

modules/ui/modeline/+light.el

@@ -368,7 +368,7 @@ Requires `anzu', also `evil-anzu' if using `evil-mode' for compatibility with
              ;; In case the user saves the file to a new location
              after-save-hook
              ;; ...or makes external changes then returns to Emacs
-             focus-in-hook
+             doom-switch-frame-hook
              ;; ...or when we change the current project!
              projectile-after-switch-project-hook
              ;; ...when the visited file changes (e.g. it's renamed)

modules/ui/vc-gutter/config.el

@@ -118,8 +118,9 @@ Respects `diff-hl-disable-on-remote'."
           :n "{" #'diff-hl-show-hunk-previous
           :n "}" #'diff-hl-show-hunk-next
           :n "S" #'diff-hl-show-hunk-stage-hunk))
-  ;; UX: Refresh gutter on ESC or refocusing the Emacs frame.
-  (add-hook! '(doom-escape-hook doom-switch-window-hook) :append
+  ;; UX: Refresh gutter in the selected buffer on ESC, switching windows, or
+  ;;   refocusing the frame.
+  (add-hook! '(doom-escape-hook doom-switch-window-hook doom-switch-frame-hook) :append
     (defun +vc-gutter-update-h (&rest _)
       "Return nil to prevent shadowing other `doom-escape-hook' hooks."
       (ignore (or inhibit-redisplay