Move the "Remove" button which was customly implemented
from the ExtBoxWithBtns_Impl to the new row. This should
also solve some accessibility issues.

Also wipe some useless code which implements custom
tab behavior.

Change-Id: I602fcf23631498145d8b9ead2936ee549caf3f0d
Reviewed-on: https://gerrit.libreoffice.org/30867Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>