Provide more choice when manually creating a new Navigation Menu

[getdave]

When users create a new Navigation Menu via the block (see Inspector Controls on the List View tab) they are immediately given a new Navigation Menu.

This menu is empty - which can be confusing or undesirable (although this is highly dependent on use case and the user).

I propose that in this specific scenario we provide the user with a choice about what "new" means. For example, we could show a Popover/Modal with options to include:

1. Blank menu
2. List of my Pages
3. Existing Menu.

This would provide the user with more choice and help to avoid the scenario where menus are accidentally created (something that is regularly reported as an Issue).

Please note that we would not have this behaviour on block insertion. This should remain a frictionless experience.