[launcher] Fixed cases where panel hiding was breaking:
- not slide in/out if delta was manually set to a value different from the start value of the animation
- not slide in/out immediately if a slide out/in was already in progress
These are solved by changes in Unity2dPanel:
- split up m_slideAnimation into 2 different animations, one for sliding in, one for sliding out.
- clamp delta to be between 0 and minus its size.