Extract combo box common parts to super class to be able to reuse it in combo boxes that should not allow store related configurations

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Add support for named providers of List<SelectableEntryXO>

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Merge pull request #140 from sonatype/named-providers

REVIEW: Named providers