[NuGet] Make NuGet menus easier to discover.

How to add NuGet packages to a project is not that easy to discover
since the menus do not refer to NuGet.

Changed the following NuGet menus so they use NuGet in the label to
make it easier to find:

Main Project menu:

    Add NuGet Packages
    Update NuGet Packages
    Restore NuGet Packages

Solution context menu:

    Update NuGet Packages
    Restore NuGet Packages