Add an empty line after @newin where it's missing

* gdk/src/device.hg:
* gdk/src/display.hg:
* gdk/src/rectangle.hg:
* gdk/src/screen.hg:
* gdk/src/window.hg:
* gtk/src/application.hg:
* gtk/src/builder.hg:
* gtk/src/entrybuffer.hg:
* gtk/src/iconview.hg:
* gtk/src/listbox.hg:
* gtk/src/listboxrow.hg:
* gtk/src/pagesetup.hg:
* gtk/src/papersize.hg:
* gtk/src/printsettings.hg:
* gtk/src/radiotoolbutton.hg:
* gtk/src/toolitem.hg:
* gtk/src/uimanager.hg:
* gtk/src/widget.hg: Add an empty line after @newin where it's needed in
order to avoid bad side effects in the documentation. Doxygen assumes that
@newin is followed by a paragraph that describes what is new.