new feature: keep list open for multiselects.

[PPP01]

Hi, I have implemented a feature that leaves the list open when multiselecting, so the list does not have to be reopened all the time. You can enable it with the new option: keepListOpen

It is based on 5.0.0-beta.11.

[Edward-Ta]

This would also be helpful for debugging purposes as when you try to inspect an open list it just closes because the list div gets removed from the dom.

[PPP01]

This would also be helpful for debugging purposes as when you try to inspect an open list it just closes because the list div gets removed from the dom.

Unfortunately, this does not help at the moment, as the menu is supposed to close on blur. But I know what you mean! I use a breakpoint for debugging.
We could set up another parameter, so that the menu remains open even with blur.

[PPP01]

@Edward-Heales I did another PR for the debugging/styling purposes.

[rob-balfre]

Thanks, see (#420)