Fix disabled package sources being used in Manage Packages dialog.

Selecting All in the package source list was searching across all package
sources including disabled ones.
Show package sources drop down in updated packages tab.

Allow user to choose which package source when looking for updated packages.
Remove All from package source combo box when only one source.

Disabled package sources were being counted when deciding whether to add All
to the combo box in the Manage Packages dialog.
Always show package sources in Available and Updates tab.

Do not hide the package sources combo box in the Manage Packages dialog
when only on package source is defined.