Add Menu depth setting for top-bar

Problem/Motivation

I'm using the top-bar and I want to control how many menu links will show. My main menu will have many links and probably 5-7 levels of depth. I will use a sidebar menu to show the deeper menu items that don't show in the top-bar. I just don't want to show too many menu links because I'm not a fan of that many drop downs.

Proposed resolution

Create a new setting that allows people to choose how many menu links they want. The default will be all of the menu links, but people can choose anywhere from 0 (all) to 10 levels deep. (could be more if needed, but 10 is a nice number)

Remaining tasks

• Patch upload
• Patch review

User interface changes

New select option in the theme's setting. It is nested with the other Dropdown menu settings and under the Menu text setting.

API changes

none