Allow dragging the divider in vtable

Allow dragging the divider in vtable

* lisp/emacs-lisp/vtable.el (vtable): Add a keymap cache.
(make-vtable): Allow dragging the divider.
(vtable-insert): Don't put the table keymap over the entire line
-- avoid the divider, which has its own keymap.
(vtable--drag-resize-column): Adjust to the in-buffer divider
dragging.