glretrace: Limit duplicate messages.

Of any kind, and not just
gui: add wglGetCurrentContext to default filter
Display thread id in GUI
gui: Tweak thread id presentation.

Match CLI, and only show on details for now.

Issue #199.
gui: print the marker string from glStringMarkerGREMEDY directly
gui: Generalize from glStringMarkerGREMEDY to all markers.
gui: Silence duplicate widget warning.