[NuGet] Use new status icons for packages in Solution window.

Use the new status API to make the display of NuGet packages
(missing, not restored, or requires retargeting) in the Solution
window consistent with how the status is displayed for projects
and assembly references.
[NuGet] Indicate package is installing using label in solution pad.

When a NuGet package is being installed the package reference node in
the solution pad now includes the text "(installing)" in the label so
the current state of the package can be determined.

For example:

    NUnit (installing)
Merge branch 'solution-pad-status'