Active UI tracking

[HalfbyteHeroes]

Currently, when wanting to subscribe to the events exposed by a UIInstanceManager (like onControlsShow), the only way to do that is through the currentUi on the UIManager. However, the currentUi changes whenever the UIManager switches to a different UI variant, and there is currently no event that would allow to detect that.

This PR therefore introduces the onActiveUiChanged event which is dispatched whenever the UIManager switches from one UI variant to another. The arguments of that event expose both the previously active UIInstanceManager, should there be one, and the current one that the UI manager has just switched to.

Additionally, a public activeUi field was added that exposes the UIInstanceManager of the active UI variant.

[bitmovin-kenny]

LGTM, just one minor thing to think about.