[Material] Animate TextBox Header/Placeholder on Focus

[kazo0]

We could move the animation from from NotEmpty VisualState to Focused VisualState. It would be something like:

<VisualState x:Name="Focused">
	<VisualState.Setters>
		<Setter Target="PlaceholderElement.Foreground" Value="{ThemeResource PrimaryBrush}" />
		<Setter Target="NormalBorder.Height" Value="2" />
		<Setter Target="NormalBorder.Fill" Value="{ThemeResource PrimaryBrush}" />
		<!--Set Placeholder to Visible-->
		<Setter Target="RealPlaceholderElement.Visibility" Value="Visible" />
	</VisualState.Setters>
	
	<Storyboard>
		<DoubleAnimation Storyboard.TargetName="FocusedBorder_ScaleTransform"
						 	Storyboard.TargetProperty="ScaleX"
						 	Duration="{StaticResource MaterialAnimationDuration}"
						 	EasingFunction="{StaticResource MaterialEaseInOutFunction}"
						 	To="1" />

		<!-- Header Animation Moved from NotEmpty VisualState-->
		<DoubleAnimation Storyboard.TargetName="PlaceholderElement_CompositeTransform"
							Storyboard.TargetProperty="TranslateY"
							Duration="{StaticResource MaterialTextBoxAnimationDuration}"
							EasingFunction="{StaticResource MaterialEaseInOutFunction}"
							To="-11" />

		<!-- ContentElement TranslateY value changing depending if there is a Header or not -->
		<DoubleAnimation Storyboard.TargetName="ContentElement_CompositeTransform"
							Storyboard.TargetProperty="TranslateY"
							Duration="{StaticResource MaterialAnimationDuration}"
							EasingFunction="{StaticResource MaterialEaseInOutFunction}"
							To="{Binding Header, RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource EmptyToCompositeTransformTranslateY}, TargetNullValue=0, FallbackValue=0}" />

		<DoubleAnimation Storyboard.TargetName="PlaceholderElement_CompositeTransform"
							Storyboard.TargetProperty="ScaleX"
							Duration="{StaticResource MaterialTextBoxAnimationDuration}"
							EasingFunction="{StaticResource MaterialEaseInOutFunction}"
							To="0.7" />

		<DoubleAnimation Storyboard.TargetName="PlaceholderElement_CompositeTransform"
							Storyboard.TargetProperty="ScaleY"
							Duration="{StaticResource MaterialTextBoxAnimationDuration}"
							EasingFunction="{StaticResource MaterialEaseInOutFunction}"
							To="0.7" />
	</Storyboard>
</VisualState>

The result:

@agneszitte @kazo0 @NVLudwig

Originally posted by @erikvilima in #974