Edit warning should get value from editors

Editwarning needs to make sure that any editor that uses something
other than wpTextbox1 to keep the value of the form, is able to write
this into the form before we are allowed to leave the page.

This is already supported by the textSelection api, so use that to
retrieve the value instead of plain .val()

Bug: 61117
Change-Id: I24d7096929c4eed36376ef9089d7f1b72a0b6682
Merge "Edit warning should get value from editors"