I've been playing this some more, still not been able to beat the AI though. One thing I found annoying was that I couldn't tell what was what in the sidebar so I added some tool-tips, I notice you have that as an enhancement. Maybe you want to take a look and see if it's what you're after:Tool-tips
I've added a singleton for the tool-tip, a method to sidebar page to get a button given its coordinates and a bit to your update logic. I couldn't workout if your start page has descriptions for the buttons so couldn't make the tool-tips work for that. If it's okay I'll fork, spread the changes over a few commits and tidy it then create a pull request?
I've not used Slick2D before but I think I may have to try it next, seems a good next step.