Includes also lots of infrastructural changes, making the
conversion of the rest of the dialogs much easier.

StartExecuteAsync should be used in-place of StartExecuteModal
and the latter removed from the code-base incrementally. More
common code from Dialog::Execute should be moved to
ImplStartExecuteModal in a next step, as this is used more widely.

Reviewed-on: https://gerrit.libreoffice.org/47722Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>
(cherry picked from commit c40dfabd)

Change-Id: Idb2c1ec790e38f582438471a0419a56cdcf1439d