Support older Vaadin versions

[tarekoraby]

Since many Vaadin apps are still on older versions, we should consider adding support to those if possible. Specifically, it would be good to consider supporting Vaadin 14, and maybe also Vaadin 8 and 7.

Related to #108.

[thevaadinman]

I left an earlier comment in the same vein, but I seem to have been mistaken about the scope... however, it still looks to me like some sort of abstraction layer would be needed so that V23+, V14, V8 and V7 could somehow plug into it as modules.

Someone with more experience could probably add some helpful information here about how that sort of compatibility would actually be achieved.

[heruan]

V14 support added in #145