Add automatic opening behaviour to side panel

[lszomoru]

Testing #133919

Some notes as I have played around with the second panel:

• It took me quite some time to figure out that first I need to toggle the side panel before I can drop things. It would be great if we would detect that something is being dragged to the opposite side of the primary panel and we would open the panel and provide the visual cue to drop the view container/view.
• I could not figure out why some view containers/views cannot be placed in the secondary panel. Ideally all of them should support to be in the secondary panel. If for some reason that is not the case, we need to have a clear visual indicator of the view containers/views that cannot be placed in the secondary panel.
• I think we should have a setting the controls whether the panel is automatically hidden when the last view container/view is being dragged out. As I was playing around with things, I always had to toggle the secondary view before I could drop something.
• The pin icon in the secondary panel does not seem to be centered (this might be a timeline view issue)
  

[sbatten]

To Verify:

1. Enable side panel with workbench.experimental.sidePanel.enabled
2. Reload window for good measure
3. With the side panel NOT visible, drag a view/container from the activity bar/side bar/panel to the side of the editor where the side panel should be

Verify it pops out allowing you to drop the view (this should function just like the panel)

[joyceerhl]

Perhaps the placeholder text could do with some padding (though I don't know how I managed to get the side panel to show up without dragging and dropping...)

[sbatten]

thanks @joyceerhl created and fixed with #135970