Use FQN instead of import for the bad Element class (#13287)

This is the first step towards eliminating usage of the class. Reasoning
about upcoming steps will be easier when it's clear which class
"Element" refers to.

Change-Id: Iea5547f789edc77c3b4ac95a5e2eb34d2717999e
Fix formatting in vaadin-checkstyle.xml

Change-Id: Ie141540d8cbad4c350c28662d1f71b6a179bbeed
Trigger a checkstyle error if importing the wrong Element class (#13287)

Change-Id: Ief15e88c9c6fe5fd88dbf9f12783ac2f5ae617c6
Refactor to use the right Element class (#13287)

Changed implementations and APIs to use the non-deprecated Element class
wherever possible without breaking backwards compatibility.
* Methods defined in interfaces have not been touched.
* Return types have only been changed methods that should have no
existing third party callers (i.e. private, internal or @since 7.2)
* For methods that third party code might have overridden, the method
has been deprecated in favor of a new method that just delegates to the
old method.
* For methods that can't reasonably be overridden by third party code
(i.e. private, final, static, internal or @since 7.2), the parameter
type has been changed without retaining the old method.

Change-Id: I7da943a77b8d0d0b9185d8c70f87d676a275d24b