Profiler: Parse and render signpost strings

The first perf_event argument to a PERF_EVENT_SIGNPOST is now
interpreted as a string ID (in the profile strings set.)

This allows us to generate signposts with custom strings. :^)

Userland/DevTools/Profiler/Profile.h

@@ -178,7 +178,7 @@ public:
         bool in_kernel { false };
 
         // FIXME: Put event type-specific arguments in a union to save memory.
-        FlatPtr arg1 {};
+        String signpost_string;
         FlatPtr arg2 {};
 
         Vector<Frame> frames;