UI: Action changes to updating previous version after installing

[jeffhandley]

After installing the latest version of a package, the following happens:

1. The Action changes to "Update"
2. The Version changes to the previous version
3. The button changes to "Update"

When selecting a package that is already installed, here's what I'm thinking we should do:

1. If there is an update available, select the latest version (respecting prerelease checkbox) and set the action to Update.
2. If the latest version is already installed, set the action to Uninstall.

[jeffhandley]

Ended up breaking apart the Upgrade/Downgrade choices for project-level management, while retaining the Update option for solution-level management.

For a project, it's now a clear separation between Upgrade and Downgrade, and the action ordering urges you to upgrade first, but if there are no upgrades available, then uninstall is selected rather than downgrade.

For solution-level management, the distinction between upgrade and downgrade wouldn't really work because a new version could be an upgrade for one project but a downgrade for another. So I left it as just Update at the solution level.