Moved MenuItems to structs/menu_items

-Moved PauseMenuItem.h, MainMenuItem.h and MenuItem.h to
structs/menu_items
-Updated all references to these headers in MenuManager, Renderer and
MenuLits
Moved game objects to structs/game_objects

-Moved all game objects ( innherriting fro GamePiece ) to
structs/game_objects
-Updated includes in BoardLoader.cpp, GameManager.cpp, Renderer.cpp, Renderer.h, TilePosition.h according the new location of game objects
Moved TCPMessage and TCPConnection

-Moved TCPMessage.cpp/.h and TCPConnetion.cpp/.h to structs/net/
-Updated references to the header files
-Updated TCPMessage.h with new locations for the enums
Added DXBall.pro

-DXBall.pro is a Qt project files. It's an easier way to generate a
makefile.
-buildDXB.sh already uses this file to create a makefile that in turn is
used to build the project.