make the application to support different resolution and "full screen"

[allan-simon]

for a first shot i guess the idea would be the following:

1 - define a view with a given size (let's say 800*600, which is current setting)
2 - we draw everything in that view
3 - we paint the "main" application in same background color
4 - we position the "game" view and size so that is centered nicely (or take full screen if it's a 4/3 ratio screen) and "strech" the view to be as big as possible
need to make some research on how to implement that idea in practice

[allan-simon]

https://github.com/LaurentGomila/SFML/wiki/Source:-Letterbox-effect-using-a-view