Add `frida_unref()` helper API for synchronous bindings

The non-POD objects (DeviceManager, Device, Session, and Script) must be
released using `frida_unref()` in bindings that don't integrate into the
GLib main loop. This ensures that the object is released in the Frida
thread.

However, POD objects (DeviceList, Device, ProcessList, Process, and Icon)
should still be released using `g_object_unref()`.