4. Added text to buttons

-Simplified init() in Main.cpp

-Added library SDL_tff for using fonts

-Added initiation of SDL_tff in init() in Main.cpp

-Changed how the menu and buttons work. Adding buttons to the menu now
requires a text to be displayed on the button, text size and text color
in addition to other parameters.

-Some other changes done in Menu.h, Button.h and Texture.h.

-Buttons now dont have a Rectangle to store Textures, instead it has its
own pointer to a Texture.
-Few other minor changes