fix transient dialog warnings on workspaces
Noticed through one of the issues reported with mutter!98 (merged). If you:
- Open gedit, make the window big enough to comfortably fit the close dialog inside
- type something
- Open the overview
- Hit the close button of the gedit window
- Hit it again
- And again
You see warnings around the close dialog being destroyed and created again. The two patches fix these.