Fullscreen option can be in conflict with user settings

[JulienPeloton]

Hello,

Starting from v3.3.0, the FullScreenActionButton sets the property zIndex to 1 in full screen, and de-reference it otherwise.

If Aladin Lite is embedded in an application that uses a different zIndex logic to position some of the elements above others (such as a navbar), this results of the Aladin fullscreen being below some elements... :-(

I propose to remove explicitly the lines 63 and 78 and let the user sets its own value for aladin.aladinDiv.style.zIndex if need be (tested successfully locally with versions 3.3.0 and 3.4.5).

[tboch]

@bmatthieu3 : what was the rationale behind this change? Can you try and assess Julien's suggestions?