Add a new settings page that lets the user choose whether admin menu settings are per-site or network-wide, as well as specify who can access the plugin. WIP; needs testing.

git-svn-id: https://plugins.svn.wordpress.org/admin-menu-editor/trunk@769061 b8457f37-d9ea-0310-8a92-e5e31aec5664
Minor: Remove unused settings key.

git-svn-id: https://plugins.svn.wordpress.org/admin-menu-editor/trunk@769063 b8457f37-d9ea-0310-8a92-e5e31aec5664