Return a full reference when parsing triggers

Return a full reference when parsing triggers

We're not returning a full reference for GtkNeverTrigger, but we are
returning full references for mnemonic and keyval triggers; this means
we're either going to leak mnemonic and keyval triggers if we consider
this function a "transfer none" one, or we are going to trigger an
assertion failure when finalizing a never trigger, if we consider this
function a "transfer full" one.

Let's be consistent, and always return a full reference to the caller.