[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.