Test: View scalability

[bpasero]

Test for #15155:

• Windows - @seanmcbreen
• OS X - @cdias
• Linux - @bgashler1

Complexity: 4

In order to support scenarios where extensions contribute viewlets to the activity bar, some work was done to enable scalability in the activity bar.

Setup:

• get https://github.com/Microsoft/vscode-extension-samples/tree/master/tree-explorer-sample
• npm install
• run it from VS Code as extension

Verify

• you can show the contributed viewlet by
  • using the new View > Additional Viewlets entry
  • right clicking on empty space in activity bar and selecting it
  • by using the new view picker (quick open -> type "view " with a trailing space)
• if you make the window small enough, viewlets that do not fit the space move into a new "..." overflow menu
  • entries in the overflow menu still indicate activity as decoration to their label
  • the current active viewlet is indicated in the overflow menu if inside
• you can move viewlets around via DND and the position is remembered between restarts
• you can remove viewlets from the activity bar and bring them back by
  • unchecking them from the activity bar context menu
  • right clicking on the viewlet and pick the related action to remove
  • removed viewlets still show up at the end of all viewlets when they are active but hide once another viewlet opens
• you can bring back a removed viewlet either by
  • using the context menu on the empty space of activity bar
  • by opening it and from the context menu select the related entry to keep it

Verify some corner cases where you combine:

• overflow of viewlets so you see the "..."
• reordering of viewlets
• keeping/removing viewlets

Watch out for crashes in the console in that case, there may well be some layouting bugs 👍

[seanmcbreen]

Looked good to me found some minor issues attached to this.