Improve custom funkeys user experience

[NanderTGA]

When you open the custom funkeys form and RadicaGame/CustomFunkeys doesn't exist, an uncaught error will occur and the user will think this menu doesn't work when it does.
Showing a popup saying the directory doesn't exist also doesn't seem like a proper solution.

Let's remove the listbox and put a label that tells the user to add their custom funkeys to that directory.
We should also make the help box a popup like the options and about ones and write a better explanation on how the custom funkeys work.

[NanderTGA]

I have an easier solution: when RadicaGame/CustomFunkeys doesn't exist, we create it and put an example file in there.
And also clarify some things in the help window.