Started moving from Gunslinger's GameStateManager to ZED's GameStateManager

|BUILD CONFIGURATION|
[Linux/Makefile]
* Added the ZEDGame library to the list of libraries to link against

|SOURCE|
[Common/GameStatemanager]
* Functionality inherits from ZED::Game::GameStateManager
* Initialise must be called after the renderer is set
[Common/GameplayGameState]
* Functionality inherits from ZED::Game::GameState
* Exit calls the GameStateManager's Quit function
[Common/Game]
* Replaced all instances of events with ZED::Utility versions
* Pointer is only warped to the centre of the window if the co-ordinates differ
  from the previous frame