we do not use CEGUI anymore, we now use libRocket as it was an easier interface designer and is based on the HTML / CSS languages. As Tattie says, there is a good example for the pearl diver game as a main menu.
Game States are a much more modular coding development. All you simply have to do is program the specific states to do what you need and switch them out. But it isn't just Main menus or pausing where they can be used. They would be great for things such as AI and more complex behaviors. There are a few custom user states which you can use however you want.
If i were you though, i wouldn't use the states until you are comfy and back in the flow of things as they are a more advanced system of programming.
The other things the states stop is constantly having to check in update loops whether the game is doing something or not, e.g. return if paused in the update loop, Disable player if "whatever" happens.
For example with states, if it is a paused state, then only the code in the paused state functions will play skipping all the rest.
States each have their own Enter, Exit, updates, mouse events, key events etc...
This tech demo may help you with states: "Asset Library\Scenes\Tech Demos\Misc\User States_A1"
The GUI Editor loads and can edit files. In in the HTML script, you can enter a lua function name to call if you are clicking a specific button or moving a slider. This makes it much easier to program.
Here are a couple of GUI tech demos to help:
- "Asset Library\Scenes\Tech Demos\GUI" (anything in that folder)
- "Asset Library\Scenes\Tech Demos\Lua\Lua - Time Of Day" - this changes the time of day based on the sliders input which calls through to lua.
i completely agree with taking the challenge and try to get as far as you can on your own, i find its often the best way of learning. But we are all here to help when you need it
C++ will be added in when the demand is met. It is a lot of effort and a core change to skyline.
@Tattie, we will end up having all sorts of AI in the end. We have been talking internally about having the different scripts for a sheep roaming and grazing to birds flying and flocking, to AI shooters such as the Ork. We find it difficult though to find the time to add game mechanics when we are constantly evolving the engine. But at some point the tables have to turn and we end up focusing on mechanics more than the engine lol
Skyline has got to be the best i have used and the first one i have seen where you can make a character the player rather easy if the PC Animations were increased to allow more and a option to make it an AI or Character it would be amazing and would be something no one has.
Thanks
The "Player character" now has the ko animation added from the current release. And i think StarFire mentioned something about adding the Reload anims to the FPS system or was it apart of the weapons editor. I think she can explain it better as the FPS is her dev area. ( We don't cross each others boundaries much lol
It allows us to develop faster. )
As for AI, Shando has already used the Ork AI as a base and expanded on it, he changed the model and edited the animation in the script and from his videos it looks like it works. Characters need to be 1:1 ratio to work correctly though with the character controllers.
now I feel old.. lol
one i wouldn't mind paying for 
i know some lua but not enough for the AI task.
Or program a 6800 chip in hex [machine code].That brings a chill down my spine! I am so happy that today we have the better/easier C++ and a big IDE; feels more like a pleasurable process. 
As SP mentioned there is a help page inside the gui editor with a few clues and a simple API for the events etc.
In fact last year, I released a version of Yahtzee for the ZX Spectrum's 30th Birthday written entirely in Z80 machine code 
I would have released it in 2012, but it took me ages to get the computer player to a state where it could play a reasonable game 
but will be back in Perth tomorrow and should have some time to play tomorrow night