It's time for new releases of glibmm

[Kjell Ahlstedt <kjellahlstedt gmail com>]

glib 2.60.0 has been released. Therefore it's time for new releases of glibmm and yet another new ABI name in the very unstable master branch in the git repo. This is what I'll do unless someone has a better idea:

Master branch
New ABI name: glibmm-2.62
New unstable release: 2.61.1

glibmm-2-58 branch
New stable release: 2.58.1
Then create branch glibmm-2-60 from glibmm-2-58

glibmm-2-60 branch
Implement issue #37 Gio::DBus::Proxy::property_g_flags() broken
Add necessary Glib::Value specializations for a few other properties (as in master).
Implement issue #33 Add custom class init and instance init functions
Deprecate Gio::TlsConnection::set/get/property_rehandshake_mode()
New stable release: 2.60.0

I'll generate documentation for the tarballs with Doxygen 1.8.11 this time. I think that's the last Doxygen release that library-web is compatible with. https://gitlab.gnome.org/Infrastructure/library-web/issues/12