Additional work on new InputMapping feature

* Rename nom::Action struct to nom::InputAction.

* The InputMapping typedef is now a std::multimap; this allows for an action to
have multiple input events linked to it (one-to-many relationship).

* Introduce nom::InputContext class (Action to State Mapping).

* Introduce nom::InputMapper class; this replaces the input manager that was
previously in nom::SDLApp.
Rename system/events/Action to system/events/InputAction
EventHandler: Introduce nom::Joystick class

* Move Joystick related code into a separate class.
SDLApp: Comment out InputContext
InputContext: Introduce clear method
InputContext: Implement removal of existing contexts before adding

This fixes the same segfault issue we have in InputMapper.
Clock: Introduce ticks_as_string function