Sort the "Target page" dropdown in alphabetical order, with top-level menus at the top.

git-svn-id: https://plugins.svn.wordpress.org/admin-menu-editor/trunk@941626 b8457f37-d9ea-0310-8a92-e5e31aec5664
Change the default title of the "< None >" option to "Unclickable Menu" because that's what it is.

git-svn-id: https://plugins.svn.wordpress.org/admin-menu-editor/trunk@941627 b8457f37-d9ea-0310-8a92-e5e31aec5664