apps-menu: Add missing chain-up

PanelMenu.Button is a bit weird in that it also "contains" its parent
actor. That container is supposed to be destroyed with the button, but
as we currently don't chain up to the parent class' _onDestroy(), we
leave behind an empty container every time the extension is disabled.

Fix this by adding the missing chain-up.

!75


(cherry picked from commit dfeb99fc)
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