Fixes#17728 by setting the standard buttons in the
button box instead of manually adding them by code.
This way the buttons should honor the platform-specific
position and look&feel.
This modification was necessary because the current implementation
of the source select dialogs within the unified add layer dialog
create the provider dialogs the first time and do not destroy
them, this means that the canvas extent and CRS can change from
a dialog invocation to the next and the extent and CRS need to
be updated at layer creation time.
