Fix duplicate symbol linking err
Input: Disable virtual event handler method on_window_resized

TTcards ends up in an infinite loop when this virtual method is called.
Input: Change back all instances of Event& to SDL_Event*

This is (hopefully!) a short-term fix until I figure out how to get
around passing onwards the SDL_Event struct onto our game states as an
nom::Event.
SDLApp: Fix seg fault that occurs during game state jumps

* SDLApp::add_input_mapping(): Remove existing input mapping key when
existing.

* Introduce EventCallback::valid method.
SDLApp: Receive input mapped events last in SDLApp::on_event method

This arrangement allows us to use custom nom::Input event handlers along
side our input map -- events were being lost otherwise!
system/Sleep: Relocate functions to system/clock.[cpp|hpp]
CMakeLists: Re-add Property Tree header files (for IDEs to pick 'em up)
EventCallback: Introduce get & execute methods
Input: General clean up

system/events/EventQueue.hpp has been removed; never used.
UserEvent: New features -- constructors, independence from nom::Event.
EventDispatcher: Refactoring of class

* Removal of the UserEvent enumeration; these are best left defined
outside of nomlib.

* Method argument signatures now use the nom::UserEvent struct.
* Method return signatures are now boolean.
* SDL_UserEvent's data2, timestamp & windowID fields is now used.
examples/events: General clean up & addition of nom::EventDispatcher
Relocation of EventDispatcher to system/events
Rename of nom::Input to nom::EventHandler
examples/events: Remove on_key_down method
Action: Introduce clear method