Now, when the user clicks on a tab in the DataViewport, the corresponding Editor becomes
active. Likewise, when the user selects an Editor with a Visualizer, the corresponding
tab becomes active in the DataViewport.

Previously, if you placed a sink in front of a source, it would lead to
a never-ending "while" loop. By removing two lines (254-255), this problem
was solved.

Tabs now work reasonably well when right-justified.

Border is now a square (except for upper-right corner).