When we are going to open new image windows, unset the transient
window. We don't need it since we will use gdk_window_raise() to
keep the dialog on top. And if we don't do it, then the dialog
will pull the image window it was invoked from on top of all the
new opened image windows, and we don't want that to happen.

Patch heavily inspiried by hack from Massimo Valentini.

Also add a manual test case for the fix. (I don't think there is GTK+
API to automate it).