Improve the Swapping Experience

[remlap42]

Add a Swap button beside the Join button
List the calls on the left side of the window (in a stack -- if name is available use the name, if "Unavailable", use the phone number)
Clicking Swap implicitly holds the active call and activates the held call
the active call should appear bold or emphasized -- the inactive calls should be greyed or deemphasized.