tdf#123937 XML Filter Settings Dialog: Move "help" and "close" buttons

Move the "help" and "close" buttons to the bottom of the dialog.
The "help" button will be at the bottom left.
The "close" button will be at the bottom right.

Change-Id: I496a3c95dd4761d8a360ad85fc5af03d0c53f131
Reviewed-on: https://gerrit.libreoffice.org/69468
Tested-by: Jenkins
Reviewed-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>