Making custom widgets work in Gtk.Builder

[martinKupec]

This series of commits is supposed to fix and enchant the use of Gtk.Builder with custom widgets.

It enables to pick GLib type name of custom class. It also fixes old problem with pairing native and managed objects during class creation.

There is problem with Gtk.Builder with gtk+ > 3.12 as it newly check for existence of signals during parsing phase. This is fixed by properly creating native signals for custom widgets.

[monojenkins]

Hello! I'm the build bot for the Mono project. I need approval from a Mono team member to build this pull request. A team member should reply with "approve" to approve a build of this pull request, "whitelist" to whitelist this and all future pull requests from this contributor, or "build" to explicitly request a build, even if one has already been done. Contributors can ignore this message.

[wolfgangmauer]

may this help, without patch glade_gtk-sharp_custom_widgets