highlighting UI items

I've implemented a mechanism to "highlight" UI items. This can be used whenever we wish to draw attention to
particular UI elements. As of now, I'm using it to highlight save slots.
 
I'm not sure if the position of the Highlight method is correct in the inheritance tree. It might be better if it's
moved to Choice. Or it could also be implemented separately, so that classes that want to enable highlighting can extend from that class. I implemented the easiest solution I could think of.   

![screenshot_2013-10-07-23-11-12](https://f.cloud.github.com/assets/1694861/1282738/7fd972ec-2f79-11e3-9c7d-fa2e85471ae6.png)

The light blue coloured save slots are those save slots that have save data stored in them. Those changes will have to be merged into PPSSPP. However, it depends on the Highlighting mechanism. So, I'll create that pull request once this is done correctly.