glib.Value dereferencing will finalize

[diamondburned]

Right now, pkg/core/glib.Value does not have the extra layer of indirection
that other structs have for finalizing, and as a result, dereferencing a Value
upon construction will cause it to be finalized early.

type Value struct {
	gvalue C.GValue
}

func NewValue(v interface{}) *Value

v := *NewValue("string")
// Underlying value might be finalized.