[NuGet] Fix updated package version missing for unrestored packages.

Package updates are checked even if automatic package restore is
disabled. Previously if the packages were not restored, and there are
updated packages for a project when it was opened, the number of
updated packages was shown on the Packages folder in the Solution
window but the updated package version was not displayed.

Now unrestored packages and packages requiring retargeting will be
displayed in the Solution window with orange text and a warning
icon but will also have the updated package version number displayed.
Merge branch 'nuget-monodevelop-5.2' into nuget-support-v2
[NuGet] Check for updates after adding packages to new project.

Fixed bug 21013 - Package update information is neither showing on
Packages folder nor showing next to package name.
https://bugzilla.xamarin.com/show_bug.cgi?id=21013

The solution opened event was fired before the NuGet packages were
 installed by the project template so no updates were found.

Now when a new solution or new project is created, and the project
template installs NuGet packages, the NuGet addin checks for updated
packages after successfully installing packages into the project.
Merge branch 'nuget-monodevelop-5.2' into nuget-support-v2

Conflicts:
	main/src/addins/MonoDevelop.PackageManagement/MonoDevelop.PackageManagement.csproj