Make the drag-bar for resizing tree/label panes more visible

[Robert Leach]

When "show trees" is true, the user can change the size of the label and tree panes by dragging the boundary with the mouse. It's hard to see that this is possible, so it should be made more visually apparent.

[Christopher Keil]

This is only one specific numerical setting when setting up gtrPane and atrPane in DendroView. Those are JSplitPanes which have a method called setDividerSize(int size). I have already changed it and it will be part of a future commit, so the issue can be regarded as resolved.

[Anastasia Baryshnikova]

What really helps is the little divider symbol that appears when you hover over the boundary between the tree and the labels. But that divider symbol only appears after your click "Show trees" at least once. Even if the trees are visible right after clustering, if you don't click on "Show trees" off and on again, the little symbol doesn't appear. This could be a different issue, but I'll reopen this one, just to keep things simple. Fell free to change if necessary.

[Christopher Keil]

This appears resolved now. I also added color to the actual divider bar. It's visible on Windows Look and Feel but invisible for Ubuntu (GTK) and OSX otherwise. I'll test this and tweak it before formally resolving the issue here.

[Christopher Keil]

Changing status to resolved again. The color/ size of the drag bar can be changed but it's very apparent and visible now.