Use Jackson for public API and implementation instead of Elemental

[mshabarov]

Describe your motivation

Jackson library (com.fasterxml.jackson) is more popular than Elemental (elemental.json), thus Vaadin should better use it in public API.

Describe the solution you'd like

0. Add an adapter code that converts Jackson to/from Elemental
1. Change Flow internal implementation to use Jackson
2. Deprecate existing public method with Elemental
3. Add same methods but that use Jackson
4. Remove Elemental variants in next major version.

0 can be skipped if it's easier do 1. right away.

[knoobie]

Just a note: I would suggest to implement some low level tests that Jaxkson does what you think it does.. I'm saying this because of their kinda "fast" releases the last years - which can sometimes break things.. so it might be worth to improve coverage in that area - especially with different Jackson versions in client projects and an upcoming new Jackson major (soon TM)