lib: add `extraClass` property to cockpit-components-select
lib: add `extraClass` property to cockpit-components-select

Allows CSS classes of the component to be extended by calling code.

Reviewed-by: Peter <a316049dd34607c62d4611d63ab22a6454add223@redhat.com>

garrett/cockpit,stefwalter/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,stefwalter/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,mareklibra/cockpit,andreasn/cockpit,moraleslazaro/cockpit,petervo/cockpit,martinpitt/cockpit,deryni/cockpit,petervo/cockpit,garrett/cockpit,cockpituous/cockpit,martinpitt/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,garrett/cockpit,andreasn/cockpit,stefwalter/cockpit,cockpit-project/cockpit,garrett/cockpit,mvollmer/cockpit,cockpituous/cockpit,Scribery/cockpit,deryni/cockpit,cockpituous/cockpit,SotolitoLabs/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,mvollmer/cockpit,andreasn/cockpit,cockpit-project/cockpit,stefwalter/cockpit,stefwalter/cockpit,martinpitt/cockpit,cockpituous/cockpit,mareklibra/cockpit,andreasn/cockpit,moraleslazaro/cockpit,deryni/cockpit,andreasn/cockpit,deryni/cockpit,cockpit-project/cockpit,SotolitoLabs/cockpit,SotolitoLabs/cockpit,Scribery/cockpit,mareklibra/cockpit,mareklibra/cockpit,petervo/cockpit,Scribery/cockpit,andreasn/cockpit,petervo/cockpit,petervo/cockpit,garrett/cockpit,mareklibra/cockpit,mareklibra/cockpit,deryni/cockpit,Scribery/cockpit,cockpit-project/cockpit,SotolitoLabs/cockpit,SotolitoLabs/cockpit,SotolitoLabs/cockpit,Scribery/cockpit,Scribery/cockpit,SotolitoLabs/cockpit,martinpitt/cockpit,deryni/cockpit,Scribery/cockpit,andreasn/cockpit,cockpituous/cockpit,cockpituous/cockpit,stefwalter/cockpit,stefwalter/cockpit,mareklibra/cockpit,petervo/cockpit,mvollmer/cockpit,petervo/cockpit,deryni/cockpit