Change colours for buttons in Tower Info dialog

[Michal Predotka]

My proposition is:
1. For the "Close" button use blue color (the same as for "Play" button) because the player resumes the game after clicking it
2. For the unlock tower button use grey if the tower can't yet be unlocked and green when unlocking the tower is possible.
Additionally I think restricting the unlock button width based on text on it would be better then very wide current button. I don't know if it's worth messing much with the code though.