(imenu--split-menu): Use dolist and copy-sequence.

(imenu--split-menu): Use dolist and copy-sequence.
(imenu--create-keymap-2): Remove.
(imenu--create-keymap-1): Simplify, remove third argument.
(imenu--generic-function): Use dolist.
(imenu-find-default): New function.
(imenu--completion-buffer): Use it.
(imenu--mouse-menu): Use popup-menu.
(imenu--menubar-select): Return t rather than calling imenu.