Commit 2667b9f3e5 updated the extension for a shell change that
renamed _onActivate() to _activate, but forgot to also rename the
declaration of the variable that stores the original function.
Fix this to stop a warning about assignment to an undefined variable.
https://bugzilla.gnome.org/show_bug.cgi?id=745470
Rather than re-implementing the function (and risk missing improvements
like the launch animation), call the original one as if the user had
middle-clicked the launcher.
The 'launching' signal and this._onActivateOverride callback were
removed from gnome-shell in 7ecb5af587af7ed892c6cfc5af0858a2acb04905
and 7ecb5af587af7ed892c6cfc5af0858a2acb04905 respectively.
https://bugzilla.gnome.org/show_bug.cgi?id=721864
