[TASK] RTE: Move JS method getNodeByPosition

Move JavaScript method getNodeByPosition from HTMLArea.Editor to
HTMLArea.DOM.Node where it belongs.

Releases: master
Resolves: #62804
Change-Id: I3709678e8ed639eeb49daabd2cbfc46b58912554
Reviewed-on: http://review.typo3.org/33934
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>