Some GUI additions

Three patches in here:

1) Adds the thread-id in small gray brackets before each call; without this, it's hard to figure out what's going on in an app that has multiple GL contexts on different threads.  I really wanted to expand this out to actually showing the context ID, and some filtering (e.g. "show only thread N, show only context X"), but that's harder.

2) Display the string markers from glStringMarkerGREMEDY directly in the GUI, instead of showing the call.  This makes them much more readable.

3) adds wglGetCurrentContext to the default filters

Image of what this looks like is below.

![capture](https://f.cloud.github.com/assets/194965/1688419/587b2382-5e07-11e3-9a3e-8f0afa0d246b.JPG)