MDBottomNavigation

Based on the spec: https://material.google.com/components/bottom-navigation.html#bottom-navigation-specs

Works in the same way as tabs but things are named differently:

• MDTabbedPanel = MDBottomNavigation

• MDTab = MDBottomNavigationItem

Demo currently located in tabs.py

Todo:

• Active color update when primary color updates
• Padding improvements
• Demo in KitchenSink