Multiselect search

[tymmesyde]

Allow to search on Multiselect like native select element does
#329 (comment)

[sleeyax]

I think this is not as straightforward to implement as it looks, considering the current combobox is actually a div element. Not sure if we can get away with using a regular select element instead though, I know styling possibilities have improved over the years as internet explorer died of but I'm not sure what the current state of that is. If using a native select element is not an option, we'll need some JS magic to make it work in a similar (and accessible) manner.