Consistency in structure of dropdown menus in admin bar

[demetris]

I the admin bar dropdowns we now have:

{$username}     <- Duplicates first child’s target   
*   Edit my profile
*   Dashboard
*   Log out

and

Add new         <- Duplicates first child’s target   
*   Post
*   Page

but

Appearance      <- Unique target (Themes, does not exist as child)
*   Widgets
*   Menus

I think we should make it consistent throughout.

(And there is also the My Sites menu, on multisite, which follows a different logic.)

(In [17741]) Admin Bar: Add a Themes submenu under Appearance, for consistency. fixes #15931.

s/custom-header-uploads/custom-header/ was intentional, see #17263.