CA-107598: Fixed the issue where XenCenter hides the "Licensing Error" popup when running in RDC

The reason for the error dialog appearing behind License Manager dialog was that we used MainWindow as parent of the Licensing Error dialog (in ShowLicensingFailureDialog method in ApplyLicenseEditionCommand)
and there were already 3 other modal dialogs displayed one on top of each other: License Manager, Apply License and Action Progress.
 Fixed by calling the ShowLicensingFailureDialog from the command, on action.Completed event (instead of action.Run) and passing the Parent as a parameter, so that we can use the correct parent when displaying the error dialog.

Signed-off-by: Mihaela Stoica <mihaela.stoica@citrix.com>
Merge pull request #173 from MihaelaStoica/CA-107598

CA-107598: Fixed the issue where XenCenter hides the "Licensing Error" p...