[Feature] Add count for ItemGroup collection node

[jpobst]

This is extremely minor, but it would be nice if there was some way (status bar, tooltip, menu item) to see how many items are in an ItemGroup. I am often dealing with groups with 100+ items and if I had the count I could quickly see if things are changing in builds.

Example: I would like a quick way to know how many items are in ResolvedFrameworkAssemblies: