Secondary side bar should support a minimized view so it doesn't completely disappear

[petetronic]

[jmcphers]

See also: #448

[jjallaire]

This might just be that we don't fully hide it but rather show a narrow strip with icons for each pane (Session, Help, Preview, etc.). This probably wants to use the light grey background (in light modern) rather than the full on blue background of the activity bar.

[wesm]

I just wanted to add that on Linux, the layout shortcut buttons in the titlebar are absent:

So having this affordance is extra important (the only way to restore the variables pane if you close it by accident is through the shortcuts menu)

[rjake]

I seemed to have submitted a duplicate issue, #4112. I found that clicking this icon at the top does the job but I still would like something that collapses and re-expands tabs if possible.