gobject: add GSignalGroup

Much like GBindingGroup, the GSignalGroup object allows you to connect many
signal connections for an object and connect/disconnect/block/unblock them
as a group.

This is useful when using many connections on an object to ensure that they
are properly removed when changing state or disposing a third-party
object.

This has been used for years in various GNOME projects and makes sense to
have upstream instead of multiple copies.
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