chore(GlTabs): move stories up one level

What does this MR do?

This moves GlTabs stories up one level so that they are not unnecessarily nested in the tabs groups. This group used to contain stories for GlTab and GlTabs, but we recently decided to only keep stories for GlTabs, so this level of nesting does not make sense anymore.

Additionally, this simplifies how default values are passed to the stories.