Moved the "Check for Updates", and "Add" buttons
to the new row, and started moving the buttons
from the ExtBoxWithBtns_Impl to the new row.

Needed to create new public methods to be able
to update button states of ExtMgrDialog from
within the ExtBoxWithBtns_Impl class.

Change-Id: Iea784d0b7237da3f8aa05862dbf1faf5acf98655
Reviewed-on: https://gerrit.libreoffice.org/30560Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>