Make sure screenshot is copied to clipboard before closing

I noticed that after copying screenshots to clipboard after couple times stops working. At first I thought it was a problem in xfsettingsd because restarting it would solve the problem, only to start again shortly after.

Then I figured out a call to gtk_clipboard_store was missing, meaning the application closes before the image is really transferred to the clipboard. This seemed a very obvious thing to be overlooked for many years and I decided to check the code history. That function call was commented out in 9ae621cf and completely removed in 1ef8d39b, probably the referenced discussion is this one.

I would certainly have noticed this issue during all these years I use/maintain screenshooter, probably it was a really useless function call and only recently something changed in screenshooter, xfce4-settings or gtk that made it necessary. Unfortunately it's not so straightforward to trigger the bug, git bisect would be quite time consuming, for now I propose to add back the function call and observe if it breaks anything.

@Tamaranch cc'ing you in case you know of any potential change in xfce4-settings that could be related.