Add keep_edit_history_on_reload option

When this option is set, the undo stack is maintained when reloading a
document.
Add documentation for keep_edit_history_on_reload option
Add UNDO_RELOAD action
Maintain undo stack when reloading a document
Don't prompt for reload confirmation if keeping edit history

No work is lost in that case (except for the redo stack, which is
normal), so the extra confirmation step seems bothersome.