Add commands and keybindings for setting tags

Altered items:
simple-call-tree-org-tag-alist,
simple-call-tree-tags-to-string, simple-call-tree-string-to-tags,
simple-call-tree-mode, simple-call-tree-get-tags,
simple-call-tree-set-attribute, simple-call-tree-set-tags,
simple-call-tree-insert-item