Menu Customizer: add a Dashicon to the add menu section title, instead of a + in the text string. Spacing matches that proposed in https://core.trac.wordpress.org/ticket/28504 (which wouldn't add icons to sub-sections in panels).

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@942554 b8457f37-d9ea-0310-8a92-e5e31aec5664