Reject invalid keys

Summary:
PasswordDialog accepted invalid keys, which causes storing them in user's
wallet. In that case, no new secrets will be requested from user:
plasma-nm will silently fail to connect unless key is edited or deleted.

GUI: OK button is disabled when key is invalid

Reviewers: jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20900