Menus screen: add "Menus" heading and "Add New" button

[eliorivero]

This is to make the Menus screen consistent with others in admin.

The patch does the following:

• adds the standard h2 header, in this case, "Menus"

• adds the standard .add-new-h2 "Add New" button but only when it's necessary: in Edit Menus tab, when there are more than 1 menu.

• removes the link to add a new menu in natural language, next to "Edit your menu below..." and "Select a menu to edit..."

[eliorivero]

Adds the "Menus" heading and a "Add New" button when necessary.

[eliorivero]

Shows how the patch affects the Menus screen.

[SergeyBiryukov]

#28100 was marked as a duplicate.

[celloexpressions]

This was in, then out, then in, then out in 3.6. See [23810], #23770, #23641, [23641], and #23119 (unfortunately) for context and history. I'm not sure that we should go back to this format again, as the tabs consolidate the header to draw the focus down to the actionable content on the screen. If there were a menus list table, as discussed in #23119, the add new button would make sense, but this is an entirely different UI.

[Kopepasah]

Replying to celloexpressions:

If there were a menus list table, as discussed in #23119, the add new button would make sense, but this is an entirely different UI.

Themes implements the heading and add 'Add New' button without the list table. I think this would be a good addition as every other page (list table or not) includes a heading. In addition, no other page adds a link on the page to add a new item and using the 'Add New' flows more with the overall UI of the admin.