gtkplacessidebar: Fix crash when remove or rename bookmark

When a bookmark is removed or renamed, the row is removed automatically via a call to the update_places function. Since the popovers are children of the row, it crashes. Set the popovers to have sidebar as parent before, so they can be reused and not crash.

I have a duplicate merge request for Nautilus since it is using the same code nautilus!816 (merged)