UI: Improvement of the navegation

[megu-hub]

In #166 it is mentioned that there are some problems in the navegation. Improve the lobby and UI of the application

• The options of Easy, Medium or Difficult should only show if the Player vs Computer mode is selected.
• The How to Play? button is not very clear, it should also include the text to make it more clear.
• Some older users cannot see some text beacuse of the size or color.