Control presentation of Source
Map all windows into a {[windowId]: window} object
Update the list of possible tab options and select (focus on) the appropriate option.
Rtn sorted array of {option, score}
Call to replace the current display
Gets the next option in this BufferCompletionSource assuming that this BufferCompletionSource length has been reduced by 1
TODO: this ain't going to work, need to work out position based on win.tab
Control presentation of Source