Better full screen experience

[Felix-Schmid]

To make this program better suited as a screensaver, I made some improvements to the full screen mode:

• I added a keyboard prompt, which shows that one can press “F11” to enter full screen, as people who don’t look at the code might not realise that this program supports it.
• The keyboard shortcut prompts can now be hidden, because they don’t look good if you want to use the program as screensaver.
• The cursor is now invisible in full screen mode, considering it does not provide any functionality.

I didn’t update the executable, because I don’t know, how one can make the exe file runnable without adding the SFML-DLLs.