Create "+ Add new item" sub menus for each menu item

[Rafi Yagi]

For the SP-Theme, for each menu item, create a sub-menu item: "+ Add new section". When the user clicks on this, a modal pops up asking what type of section they want to add: 1) A page 2) A post 3) A category (template?) 4) An external link.

If sp-plugin is enabled and the user picks the category or template option, then a new sp-template will be created with all the components added by default. The user will then have the option to edit the template after the new menu-item/section/template is created.

The motivation for this comes from the difficulty coming from creating the site structure - after meeting with Merre, and after having working with Barbara, Jesse W. and other users, the flow of creating a new section of the site is tedious:

1. Go to dashboard
2. Go to Appearance
3. Go to Menu
4. Select the Menu you want to edit
5. Find the item you want to add to the menu (pages, posts, categories, etc). If one of these doesn't already exist, you have to backup and create it which is another fairly complicated process
6. After having added the item to the menu, you have to drag it to the place you want it to appear.
7. Save the menu
8. Repeat the process if you want the item to appear in other defined menus.