Remove Show/Hide Tree buttons and add an icon to indicate that the panel is resizable

[Anastasia Baryshnikova]

As discussed, the 2 buttons to minimize/maximize the Tree panel seem unnecessary, so let's just remove them & add a visual indicator (a few examples attached) to suggest that the panel is resizable.

Mac Email -- shows a little dot on the panel frame (this is my favourite option; we might also consider 3 dots or 3 bars -- I seem to have seen it somewhere but couldn't find an image) Mac Finder and Mac Dictionary -- don't show anything Sws_drag_divider -- I guess this duplicates the mouse pointer but will make it very clear that that's movable..

[Christopher Keil]

added an icon with 3 dots and added a subtle color change when hovering over the divider to emphasize on interactivity.

[Robert Leach]

Merged to master.

[Robert Leach]

Removing milestone: General interface (automated comment)