Customizer Menus: @todo audit

[designsimply]

Review and resolve any remaining @todos from the initial Menu Customizer plugin merge in r32806.

/hat tip celloexpressions for the list + notes started at ​https://github.com/voldemortensen/menu-customizer/issues/66 which I'm copying here for reference:

## class-wp-customize-menus.php
#### 165: @todo: do we really need this - do we need to populate the description field here? (note: copied from existing core system)
I'm pretty sure we can remove the code here, will do so tomorrow unless anyone objects.

#### 507: 'theme_supports' => 'menus|widgets', @todo allow multiple theme supports
We really need something here - Menus should be shown if menu locations exist _or_ widget areas exist, or both. We shouldn't restrict it to just menus support. May need to wait until after merge, need a decision here.

#### 511: @todo core merge remove corresponding code from WP_Customize_Manager::register_controls().
Thing for core merge list.

#### 525: @todo if ( ! $menus ) : make a "default" menu
Need to decide if we want/need a default menu. There was some discussion in #core-customize, but no consensus yet. Will wait until after merge at this point.

#### 587: @todo we need to implement something like WP_Customize_Widgets::prepreview_added_sidebars_widgets() so that wp_get_nav_menu_items() will include the new menu items
See ​https://github.com/voldemortensen/menu-customizer/issues/70

#### 1104: @todo: consider using add_meta_box/do_accordion_section and making screen-optional?
This is in reference to the available item type sections. I think we should probably wait if we decide to do it.

#### 1270: @todo Instead of throwing Exceptions, we'll have to switch to passing around WP_Error objects.
See ​https://github.com/voldemortensen/menu-customizer/issues/72

# menu-customizer.js:

#### 804: @todo handle removal when theme location is set to blank.
This definitely needs to be fixed. Shouldn't be hard

#### 1158: @todo replace this with wp.ajax.post()
This will be addressed by #67. Ajax requests need to be eliminated entirely.
*See #67*

#### 1190: @todo this requires cloning all direct children, which will in turn recursively clone all submenu items - works, but is there a better approach?
Not good, but works for now. Type of thing that would likely be fixed in a beta if it brings up major issues anywhere.
*Cloning needs to be eliminated. This will be addressed by #67.*

#### 1195: @todo There should really be Customizer settings that contain all of the menu item fields
Yes, but is that something we can change at this point?
*Yes: #67*

#### 1353, 1355, 1371, 1373, 1410
We need to fix some issues with reordering menu items and submenus. Well, two major issues, each for up and down. It's doable but probably a lot of code.

#### 1924: @todo: Instead of {}, the initial setting should have an ID, title, and menu_item_parent
It works, but ideally we'd fix this.
*See #67*

#### 2063: @todo core this shouldn't be necessary.
Latest patch on #30737 should fix this

#### 2080: @todo core should do this for us
#30741 would fix this, among other things.

#### 2093, 2094
Need #30738 to fix this (adding missing controls to new menus)

#### 2112: @todo should we focus on the new menu's control and open the add-items panel? Thinking user flow...
Discussed this with @designsimply, I think we'd like to try testing both ways with new users. Can wait for now.

#### 2150: @todo core there should be API methods for deleting sections.
I think there are now? Let's use them (this was pre-4.1).

[netweb]

Moving into 4.3 milestone to track.

[ocean90]

Sadly, this ticket is not helpful.

The points should be dicussed on Slack first and then we can create new tickets, one for each task.

[designsimply]

Sadly, this ticket is not helpful.

:) no problem! See you in Slack.

[markoheijnen]

For #### 525: @todo if ( ! $menus ) : make a "default" menu see #32672. The current situation makes it impossible to create a menu when there is no existing one.