Rename @since back to @newin
Rename @since back to @newin

* doc/reference/Doxyfile.in (ALIASES): Rename the @since alias
back to its old name @newin, but otherwise keep the parameters.
Apparently there is a @since command predefined in Doxygen which
I hadn't noticed.  It does seem to get overridden silently, but
it is better not to rely on this.

GNOME/librsvgmm,GNOME/librsvgmm,GNOME/librsvgmm