gmmproc: Improve the conversion of Since to @newin

* tools/m4/enum.m4: Add a blank line in the enum documentation.
* tools/pm/Output.pm: Don't always add a blank line in the enum documentation.
* tools/pm/DocsParser.pm: Improve the rules for conversion of Since to @newin.
Add an empty line after @newin where it's missing

* gio/src/action.hg:
* gio/src/cancellable.hg:
* gio/src/charsetconverter.hg:
* gio/src/mount.hg:
* gio/src/notification.hg:
* gio/src/simpleaction.hg:
* gio/src/socket.hg:
* glib/src/datetime.hg:
* glib/src/glib_docs_override.xml:
* glib/src/keyfile.hg:
* glib/src/variant.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.