Add a button to add additional files

It's not wired up to anything yet
layout changes

Make the border around the top tbs wider and darker

Add a border around the enture file div and give it a light grey background
next to the top tabs
Make the add-file button place itself to the right of the tabs
Make the add watch expr button less ugly

Instead of looking like a full button, now it's just the +
Make the close-watched-expr button look better

It's still finiky - the X icon is only centered when the span size is 18 :(
give the active stack tab a shadow to make it more prominent
Complex data structures can collapse with double-clicking