settings: move settings library files into separate folder

With the new settings system I'm trying very hard to keep the actual settings library and the integration into xbmc as separated as possible. This makes it easier to localte, isolate and fix bugs and keeps a clean architecture.

To make that separation more obvious I'd like to move all the files belonging to the settings system into a "lib" sub-folder of xbmc/settings. That will also make it easier for me (and anyone else working on that code) to grasp which files/classes/code are xbmc-specific and which aren't.

I've only tested this on win32 so far, but I'll test linux later. Furthermore this needs xcode project updates. I'm fine with merging it at the end of the merge window.