Add tasks to the org-brain entry in visualize mode

Add a key-binding (`C-c t`) to directly add a TODO entry to the
current entry in org-brain. This makes it possible to capture action
items about the current entry without leaving the
org-brain-visualization window.

This change depends on the capture templates introduced in the commit
before it and adds `org-brain-visualize-add-todo` and
`org-brain-add-todo` functions to enable adding tasks under a brain
entry.

org-brain.el

@@ -2726,6 +2726,33 @@ Set up the capture templates we need in `org-brain-visualize-mode'."
               org-capture-templates-contexts)
         (setq org-brain-visualize--capture-templates-registered-p t)))))
 
+;;;; Back to Visualize
+
+(defun org-brain-add-todo (entry)
+  "Add a todo item to the ENTRY.
+If called interactively, select ENTRY with
+`org-brain-choose-entry', give a preference to
+`org-brain-entry-at-pt', if any."
+  (interactive
+   (let ((def-choice (ignore-errors
+                       (org-brain-entry-name (org-brain-entry-at-pt)))))
+     (list (org-brain-choose-entry "Add a TODO item to: "
+                                   'all nil nil def-choice))))
+  (when (not org-brain-visualize-use-capture-templates)
+    (user-error "The appropriate capture templates have not been set up. Check the README for instructions"))
+  (if (org-brain-filep entry)
+      ;; Entry = File
+      (user-error "Only headline entries support adding a TODO item")
+    ;; Entry = Headline
+    (org-capture nil (concat org-brain-visualize-capture-prefix-key "t"))))
+
+(defun org-brain-visualize-add-todo ()
+  "Add a todo item to the currently active entry."
+  (interactive)
+  (if (eq major-mode 'org-brain-visualize-mode)
+      (org-brain-add-todo (org-brain-entry-at-pt))
+    (user-error "Not in org-brain-visualize")))
+
 ;;;###autoload
 (defun org-brain-select-button ()
   "Toggle selection of the entry linked to by the button at point."
@@ -2862,6 +2889,7 @@ point before the buffer was reverted."
 (define-key org-brain-visualize-mode-map "e" 'org-brain-annotate-edge)
 (define-key org-brain-visualize-mode-map "\C-c\C-w" 'org-brain-refile)
 (define-key org-brain-visualize-mode-map "\C-c\C-x\C-v" 'org-toggle-inline-images)
+(define-key org-brain-visualize-mode-map (kbd "C-c t") 'org-brain-visualize-add-todo)
 
 (define-prefix-command 'org-brain-select-map)
 (define-key org-brain-select-map "s" 'org-brain-clear-selected)