Chatty's message dialogs are attached to wrong parent window

Chatty's message dialogs ("delete account" and "failed to login") are being attached to the main window as their parent, instead of the window they're actually being called in.

This leads to wrong stacking order (you can raise the preferences window above the dialog, despite of the former one being completely inactive) that may even result in inability to go back to the dialog once phoc#4 (closed) and phosh#61 are solved.

Currently phoc on master branch doesn't do stacks at all, but this problem can be already clearly seen in Mutter and KWin by clicking on the preferences window when the message dialog is shown (the dialog should be kept on top).