This fixes 2 different bugs:
- mp.add_key_binding('c', ...) taking priority over the builtin g-c
binding.
This follows up 994a08f5a7 which fixed this within the same input
section. This fixes it across different input sections.
- mp.add_key_binding('g-c', ...) not taking priority over a c binding
defined in input.conf.
This happened because is_builtin of bindings added with
mp.add_key_binding is true though they're not actually builtin.