Consider removing or making vue-virtual-scroller optional

[serebrov]

While vue-virtual-scroller helps getting some additional performance, it is not critical and component worked fast enough without it.
The disadvantage of the virtual scroller is that it adds complex HTML structure which is harder to customize (for example, it is complex to style the scroll bar).

[briancaffey]

I have come across this issue in trying to customize the height of the scroller. I'm finding strange behavior on mobile and sometimes on desktop browsers the categories are overlapping. @serebrov thanks

[apasov]

@serebrov I support your idea. I described my situation in #57

[serebrov]

Related PR that adds virtual scroller - #20

[serebrov]

@briancaffey @apasov I've just pushed version 6.1.1 to npm with new StaticPicker component - it is the same as Picker, but doesn't use the vue-virtual-scroller.

Updated the demo app, the "Custom Search And Preview Templates Example" example uses StaticPicker.

Also added related information to README.