Auto-adjust document tab width to the available space

The width document tabs should auto-adjust to the amount of available space. Currently, tabs are fixed width, and 'underflow' out of sight on the left-hand side of the screen when many tabs are open. They also appear beneath the channel manager breadcrumb when the channel manager is active, which looks pretty bad. Instead, tabs should become less wide when all available screen space is used (similar to the behavior of browser tabs).