* gladeui/glade-project.c: Added algorythm to introspect lowest possible
version of GTK+ to depend on for a project, this helps cases where the
versioning data is missing, instead of defaulting to latest which may
cause problems for people specifically targetting GTK+ 2.12 (bug 586046).

* plugins/gtk+/gtk+.xml.in: Mark accel-groups property as since GTK+ 2.16.