individual: avoid updating in get

individual: avoid updating in get

Properties with delayed loading always went through an update check
each time the property was read. This slowed down reading
unnecessarily, because constructing the value is costly and necessary
updates are already dealt with via notifications.

Fixed by introducing a new parameter "force_update" which is true
by default (= same behavior as before) and false when used inside
get().

Closes: https://bugzilla.gnome.org/show_bug.cgi?id=688834