Button tool to switch to edit source with changes

By serialising the current DOM, and injecting it into a form we can
end up in the source editor with our VE changes converted.

Bug: 50687
Change-Id: Iafcc02a737d9c6c3a59dce1caff130d47ca25650
(cherry picked from commit 7b2cacbe57ae1b73369ccec7d1ea76cbc142e6bf)