More refactoring & clean up of the InputMapper feature

* nom::InputStateMapper: Introduce disable & activate_only methods

* Rename InputMapper to InputActionMapper & InputContext to
InputStateMapper.

* Rename of most of the public methods of InputMapper & InputActionMapper

* nom::InputActionMapper now has friend access to InputStateMapper;
nom::InputActionMapper's get method is now private.

* nom::InputStateMapper::insert method's argument map is now a reference
to InputActionMapper.

* nom::InputStateMapper::insert method's argument active is now
non-default param.