Demo update #13

[p]Update[/p]

[p]Exit to the main menu using the ESC key[/p]
[p]Improved save system (2 save slots)[/p]
[p]Updated menu UI design[/p]
[p]Keyboard shortcuts[/p]
[p]Button icons added[/p]
[p]UI tooltips[/p]
[p]Technology tree: costs and UI improvements[/p]
[p]Tree chopping animation[/p]