app/core: Use NULL for "simple" signals

Apart from being less code, this actually gives us a nice performance improvement. Up until a few years ago, if you pass NULL as the marshaller for a signal, GLib would fall back to g_cclosure_marshal_generic which uses libffi to pack/unpack its arguments. One could avoid this by specifying a more specific marshaller which would then be used to immediately pack and unpack into GValues with the correct type.

Lately however, as a way of optimizing signal emission (which can be quite expensive), GLib added a possibility to set a va_marshaller, which skips the unnecessary GValue packing and unpacking and just uses a valist variant.

Since the performance difference is big enough, if the marshaller argument is NULL, g_signal_new() will now check for the simple marshallers (return type NONE and a single argument) and set both the generic and the valist marshaller. In other words, less code for us with bigger optimizations.

In case you also want va_marshallers for more complex signals, you can use g_signal_set_va_marshaller().