[TabBar] Allow for easy customization of a selection indicator

[kazo0]

In order to achieve certain scenarios such as a custom sliding selection indicator:

Or to simply customize the look of any given selection indicator, such as the default Material TopTabBarStyle's selection indicator:

We need to expose some properties that will allow us to inject any sort of content into the internal TabBarSelectionIndicatorPresenter

Spec

Option 1 (as properties on TabBar)

New TabBar Properties

Property	Type	Description
SelectionIndicatorContent	object	Gets or sets the content of the selection indicator
SelectionIndicatorContentTemplate	DataTemplate	Gets or sets the template to be used when displaying SelectionIndicatorContent
SelectionIndicatorTransitionMode	SelectionIndicatorTransitionMode	Determines the method for which the selection indicator will move to the selected tab. Values: .Snap , .Slide

Option 2 (as a standalone ContentControl)

TabBarSelectionIndicator

Object → DependencyObject → UIElement → FrameworkElement → Control → ContentControl → TabBarSelectionIndicator

Property	Type	Description
TransitionMode	TransitionMode	Determines the method for which the selection indicator will move to the newly selected tab. Values: .Snap , .Slide

New TabBar Properties

Property	Type	Description
SelectionIndicator	TabBarSelectionIndicator	Gets or sets the selection indicator used to display the currently selected TabBarItem