Fix new Package Console in MonoDevelop 5.1 and 5.2

In MonoDevelop 5.2 there is an error that is reported in the
PowerShell console after a package is installed or uninstalled.

Moved a lot more classes into the extension addin so they can be
customised and changes in the main NuGet addin do not cause the
extension addin to break.
Fix null reference in new Package Console on install/uninstall.

Null reference occurred in MonoDevelop 5.2 since the latest code
listens for package install and uninstall events which did not have
the project set. Now the extended addin does not fire these events
when a PowerShell command is run in the console.
Fix new Package Console not showing projects.

If the solution was opened before the Package Console Extension was
opened the project drop down list was not populated.