Add a new mixin for data provider

[tomivirkki]

child of #657

https://github.com/vaadin/vaadin-combo-box/tree/data-provider-proto
(the proto might not be optimal starting point to build on)

• add tests for the feature

• create dataProvider, size and pageSize APIs

  • dataProvider should receive props and callback as arguments when the internal iron-list requires new content
    • props has page, pageSize
    • callback is invoked with requested set of items (or less) and optional size
  • size should reflect as full capacity of the iron-list
  • pageSize dicatates how many items the dataProvider is asked for per request

• internally the dataProvidershould populate filteredItems

• Throw an exception if dataProvider and items are used simultaneously

• Toggle loading property while dataProvider is waiting for new items

No itemIdPath should be needed