Fixed broken SVN Commit dialog.

Summary:
SVN commit dialog was showing incorrectly because QDialogBox buttons were not attached to the dialog window.
Also fixing memory leak on QDialogButtonBox creating.

Reviewers: #dolphin, elvisangelaccio, meven, ngraham

Reviewed By: #dolphin, elvisangelaccio, meven, ngraham

Subscribers: ngraham, meven

Differential Revision: https://phabricator.kde.org/D26115