UI: replace wxCheckTree with wxTreeListViewCtrl

[oltolm]

I wanted to get rid of the non-standard wxCheckTree in the "Graphic packs" window. I replaced it with a wxTreeListViewCtrl. I think that it looks better because you now have three states checkboxes and you can see where there are active options. The only thing that was lost are colors.

[Crementif]

I like the idea to remove some of the antiquated custom controls with new features that are provided by wxWidgets, but I think not having the colors is a pretty big downgrade in terms of usability and clarity. I don't think the middle state checkboxes really add anything, and I don't really think we should have users be able to mass-enable ALL graphic packs for a game without knowing what they're enabling. Unless it's possible to have the colors too, I don't think it's worth replacing our custom solution. But up to Exzap whether he agrees with me.

[oltolm]

You can not mass-enable options. All inner nodes are read-only. You can only change leafs.