fix: only add vaadin-tab elements to items

[web-padawan]

Fixes #150

[tomivirkki]

Hmm...shouldn't this rather filter all children by child instanceOf VaadinTab similarly to what the previous major did? New Tabs would ignore VaadinTab extensions that have a tag name different from "vaadin-tab" (not sure if this is a big problem).

[web-padawan]

Anyone who wants to extend VaadinTab is free to also override _filterItems method.

We can re-introduce this check though. There is already hasSelectedStateMixin that can be used for this purpose. Would it be better to use that here? @tomivirkki WDYT?

[moderndeveloperllc]

@web-padawan You are doing something like that here:

https://github.com/vaadin/component-mixins/blob/d5d3947b7ab0c2c6bb69e9ef46eda7cb0510f958/packages/single-selection-mixin/single-selection-mixin.ts#L16-L23

Makes sense to be consistent I guess? JM2C

[web-padawan]

@moderndeveloperllc actually, it might be good idea to export that function from there.

Or maybe I rather override that protected method in this mixin directly.

[web-padawan]

Submitted vaadin/component-mixins#43 to fix that.