settings: add descriptions area to GUI

I'm putting this up in a PR in the hopes that @Jezz_X (or whoever has the necessary graphical skills) will look into this and improve the general idea. With the new settings system we have the possibility to show a description for every category and GUI setting. Right now this happens in a very ugly text viewer dialog which only appears when pressing the Info button. This introduces a new textbox to CGUIWindowSettingsCategory which can be placed by the skinner wherever it fits best. The textbox is always populated with the description of the currently selected category or setting.

The integration into confluence is just an example on how it could look like but it at least needs some border/background texture to better set it apart from the list of settings. Here are two example screenshots of how it looks right now: 
![screenshot005](https://f.cloud.github.com/assets/75864/544560/2958344c-c252-11e2-8b41-5b91a8628b47.png)

![screenshot006](https://f.cloud.github.com/assets/75864/544565/386b17ce-c252-11e2-8c76-f3237169ce43.png)

PS: A big thanks to @MartijnKaijser for taking on the effort of copy and pasting all the available setting descriptions from the wiki.