Parallel animation

[lianee]

The sync flag allows in and out animations to happen simultaneously.

With some opposite effects (left/right, up/down), it is easy to have a slider (horizontal or vertical).

Notes:

• As this works better with display:block, this PR needs the previous PR preserving css display mode (at least for the demo)
• "sync" is perhaps not the best choice for the flag name, as it is more "simultaneous", "parallel" or "concurrent". Unfortunately, I couldn't think of any better "short" name.
• Since elements are position: absolute, the parent needs to have some height. I set the height to those of the first element, there is a lot of room to improve this.

[MrSaints]

Sorry I haven't had time to look into this. It's a relatively big change. I'll try to look into it in the coming days.