Connections have a timestamp property, which records the last time
it was succesfully activated. This is useful information to us, as
more recently used connections are likely more relevant than ones
that haven't been used for a long time (or not at all).
To make use of it in the future, expose that timestamp as a property.
Only connections support it, but we will soon provide a reasonable
value for devices as well, so add the property to the base class.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2407>
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