LPS-46694 Decouple property list panel from the diagram builder

About the last three commits (dc2c44623861e085683fca675b645e3466757565 , 5b901265891f4ec0c72d6c980bea4db121299956, 995423a97dbff67d4a90292d8f73fd51533d61c2) of this pull request:
a. I removed some icons that aren't used by AlloyUI at this time and didn't replace with font-based icons.
b. The diagram-builder needs to have its element spaced better.
c. I don't know if it's appropriate to apply them anymore given that I was remembered that the current implementations almost certainly are going to be improved soon, so I guess it was a mistake to do so in the first place.
d. Notice that the dc2c446 commit kills the separation of concerns by adding back diagram-builder / form-builder logic to the property-list component.

So I suppose that the last three commits are better left ignored.