MELS: check scroll on load + use mels-1,-2,... key

[lukebarnard1]

To fix element-hq/element-web#2916, force the checking of scroll position by calling _onWidgetLoad (might need renaming...) when a MELS is expanded/contracted.

Also use an keying scheme for MELS that doesn't depend on the events contained, but rather a simple increment index based on the order of the MELS as it appears amongst all MELS.

[dbkr]

Isn't this going to cause similar problems? ie. I see a MELS (which gets index 0), I scroll a bit until another MELS comes into view (which gets index 1), I expand the one with index 1, scroll a bit more, the first one scroll off, the one with index 1 now becomes index 0 and therefore gets recreated and therefore unexpands?

I'm unsure of any way of getting a consistent expand-state here other than keeping track of what MELS each member event was put in, then making sure the MELS containing a given event always has a consistent key.

[lukebarnard1]

Basing the key on the contained events will fail because of pagination. We can only optimise for events being paginated in/out (as with this PR), or new MELS being added by using an event ID.

If we want to optimise for both, we could control all MELS (by giving each MELS an expanded prop) from the component that contains them, but even then we'd need something to key-off of.

[lukebarnard1]

Upon reflection, swapping to MELS-1,2,3 is not better than what we already have.