Add an API call for determining if tabs are visible

I have a new plugin I created for automatically showing/hiding tabs based on the number of views in a window/group. There is a problem with it though, and that's that the user must first initialize a window to get it to work correctly, and by that I mean, they must set a new empty window to hide tabs before opening or closing any files. This is necessary because there is no API call to find out if tabs are currently visible.