Console key listener (~) overriden in SDL2 backend
With `examples/vertex.nim` in nico v0.3.2 on OSX:

The console key listener doesn't get triggered when pressing the `~` key.
This seems happen because the SDL2 backend overrides `keyListeners` ([sdl2.nim:141](https://github.com/ftsf/nico/blob/master/nico/backends/sdl2.nim#L141)) from the common backend. After commenting that line out, the console is launched as expected when pressing `~`.