"merge-by-painting" logic considered harmful

[mikeroberts3000]

Surprisingly, feedback received from our group of Summer 2013 interns suggests that the "merge-by-painting" logic is counter-productive. The interns found this logic (i.e., the logic that selects the largest segment as the keep-this-label label, and applies that label to all the other segments) confusing.

The interns claimed they would prefer specifying the keep-this-label label more directly. The interns suggested the following method for directly specifying the keep-this-label label: the first label you click on when doing a merge-by-painting stroke would be the keep-this-label label; in other words, merge-by-painting strokes would always flow from the keep-this-label label to all other labels.

To work around this issue, the interns encouraged each other to never use the merge tool, and instead use the merging functionality that exists in the split tool.