Add-on Add dropdown function to the RM tabs

[Alpha1]

Now that we can add unlimited tabs to the resource manager, it would be very useful to be able to add dropdown menus to tabs. For example: if I add a FAQ tab, but my FAQ is so big that I want multiple pages, then I could add a dropdown to the FAQ tab to show:
FAQ
|- Installation
|- Upgrades
|- Functionality

We should be able to add RM fields as RM tab drop down fields.
This allows us to do two things:

1. Add a new field to a tab as a drop down menu item.
2. Add a link to a tab as a drop down menu item.