Add view models.
Add Manage NuGet Packages menu items.
Add empty Manage Packages dialog.
Set Manage Packages dialog title text.
Add controls to Manage Packages dialog.

Use text view controls to begin with to display packages.
Hide package search and package sources.

Manage NuGet packages dialog shows the package search text box and the
package sources combo box only on the Available packages tab.
Populate package sources in Manage Packages dialog.

Package sources now added to combo box.
Selected package source in combo box updates the active package source in the NuGet.config file.