plug-ins: put the initial foreground window back after a screenshot.

This was lost in commit 966843564d. It's not a big deal since this code
path would only happen when the capture using magnification API fails,
yet we may as well make it perfect.
Also taking the opportunity to change the return type to gboolean for
the various capture functions (though it is technically the same,
semantically we were returning success boolean).
And removing a comment which had been duplicated and left at a the wrong
place.