Notebooks - Make default for Jupyter NB toolbar icon as opposed to icon + label

[claudiaregio]

This request comes from the discussion started on this PR: microsoft/vscode-jupyter#7651

The outline icon was added to the global toolbar in an effort to help discoverability. With now 8 icons in the toolbar and this being added as the last, depending on the screen size this icon is more likely to end up in the overflow menu and may reduce some of the effectiveness at improving discoverability.

For other languages too, the toolbar can get really long with the text.

[greazer]

After discussion, suggestion was to create a hybrid model. If there's enough space for text and an icon, then use it. As the window narrows, text labels are removed. If all the buttons don't fit, then the overflow is populated.

[claudiaregio]

For this hybrid model, variable explorer and outline are visible correct?