Towards an awesome user interface

[TheoPannetier]

Context

The menu and options button look okay, but a bit ugly. I found an awesome CC0 sprite pack, let's use it to make more appealing buttons!
This issue is quite open-ended, take it where you want. A good start would be to use the new sprites, maybe the font for the menu buttons. In more advanced directions, we can try to have a clicking animation (the different versions of the button sprites enable this), or even have a clicking sound.
Just don't forget to break down your plan in smaller issues write tests whenever you can!

Test

Below is a starting test, to load the new button sprites in the available game resources:

#ifdef FIX_ISSUE_724
 // (724) Button sprites are loaded and ready to use
  {
    game_resources gr;
    gr.get_blue_button();
    gr.get_green_button();
    gr.get_red_button();
    gr.get_yellow_button();
    gr.get_grey_button();
  }
#endif // FIX_ISSUE_724