Fix input manager to report only active bindings as conflicting

Bindings (even when they are global) are ignored when they are not registered in the current input_context.
Move loading the keybindings file into separate input_manager::load function,

- load a second (user specific keybindings file)
- save to that second keybindings file, keep the first keybindings file unchanged all the time.
- don't write the action name to the keybindings file.