IMAP: don't block when displaying the Subscription dialog

Summary:
The configureSubscription() slot is invoked via DBus, so using QDialog::exec
to display the dialog may cause the DBus call to time out, leading to a
suspicous warning printed by KMail ("invalid reply").

Reviewers: winterz, #kde_pim

Reviewed By: winterz, #kde_pim

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D27775