extension: Reuse original handler in keyPressHandler injection

We do need to inject our own handler to treat application-switch
actions the same way as the corresponding window-switch ones;
we can actually express exactly this without re-implementing the
entire function, by calling the original handler with a tweaked
action parameter - the resulting code is not only more concise,
but should also be a bit more robust against changes in core.

https://bugzilla.gnome.org/show_bug.cgi?id=737457
launch-new-instance: Re-use original activate function

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.