gtk_clipboard_store makes the app exit hand for 10 seconds

This is something I noticed only recently with screenshooter and I'm surprised no one has complained so far.

See this reproducer, it takes 10 seconds to exit, if gtk_clipboard_store is omitted it exits immediately but the image is not persisted to the clipboard.

It's a bit nebulous to me why this is happening, and I don't want to copy and paste LLM analysis here, but it seems a regression caused by 1557fb88 (April 2024). With this little patch, that commit is effectively reverted, the problem is gone, even gtk_clipboard_store is not needed anymore, as I understand, gtk takes care of persisting the image when the app exits.