Migrate to consistent nested components

[jscheiny]

In various places we have nested components of the form <Component.SubComponent /> and in other places we have nested components of the form <ComponentSubComponent />. Neither is particularly better than the other but we should be consistent in our components. We are planning to standardize on the latter naming convention. Using this issue to track instances of this naming convention to clean them up.

[jscheiny]

The following components are exposed as both nested and unnested:

• Navbar.Divider
• Navbar.Group
• Navbar.Heading
• Tabs.Tab
• MultiSlider.Handle