jquery.textSelection: Add setContents function

This function is specified in the commands handler below, but did not
have an implementation for the plain textarea case.

Better we make this work. A user wanted this api call so his JS
cleanup script could work consistently on both plain textarea and the
CodeEditor

Change-Id: Ib3afc505037a5b12269e55b4b130a38a3fac7823
Merge "jquery.textSelection: Add setContents function"