overrides: Add convenience g_object_set() replacement

Object.assign() can be used to set multiple properties at once, similar to g_object_set() in C code. It doesn't looks quite as nice though, so add a small wrapper that replaces the non-introspectable g_object_set().

There's no g_object_get() replacement, first because it is harder, but also not least because it is hard to beat the following anyway:

let { someProp, notherProp } = obj;