Fix the client-side '+Tab' button

[soney]

(steps to re-create: open an arboretum browser, and navigate to localhost:3000 in a separate browser; click the translucent icon in the top right corner of the page)

Currently, the "+ Tab" button calls lines 55-68 in browser_state.js, which then uses the chrome remote debugger to create a new tab. This code needs to be re-written to create a new tab by calling the createNew method in browser/js/tabs.js

This change will likely require adding a way to communicate between the two files