REVIEW: Named providers

https://bamboo.zion.sonatype.com/browse/NX-OSSF51

Add a ProviderCombobox that allows specifying the binding name of an SelectableEntryProvider.
SelectableEntryProvider is then responsible for returning the list of id/name pair to be selectable in the combo box.

Example can be seen by looking at test/demo ProviderComboCapabilityDescriptor/RealmsProvider or the real ones from sonatype/nexus-pro/pull/97.