Move backstage animations from BackstageTabControl to Backstage

[batzen]

Currently the opening and closing animations are applied to BackstageTabControl which causes performance issue due to the control constantly changing it's size during the animation.

By moving the animations from BackstageTabControl to Backstage we not only improve the performance but also place them were their properties, IsOpenAnimationEnabled and HideAnimationDuration, are attached to.

I consider this a breaking change as the animation resources are moved from BackstageTabControl to Backstage and the animation triggers are moved from xaml to code behind.

Breaking changes:

• Animation resource names will be moved and renamed
• IsOpenAnimationEnabled will be renamed to AreAnimationsEnabled
• HideAnimationDuration will be removed