Bug 1091731 - Introduce gaia-select webcomponent.

Can we now revisit this now that we're using a custom element for this? I assume we could even create an element for this behind the shadow dom? I would just prefer to use a CSS hack I think instead of an SVG if we don't need the SVG. I could probably be convinced otherwise though if you feel that SVG is the superior choice. I just feel that CSS is easier to maintain and theme when needed.