Kernel+Profiler: Capture metadata about all profiled processes

The perfcore file format was previously limited to a single process
since the pid/executable/regions data was top-level in the JSON.

This patch moves the process-specific data into a top-level array
named "processes" and we now add entries for each process that has
been sampled during the profile run.

This makes it possible to see samples from multiple threads when
viewing a perfcore file with Profiler. This is extremely cool! :^)

Userland/DevTools/Profiler/SamplesModel.h

@@ -40,6 +40,8 @@ public:
     enum Column {
         SampleIndex,
         Timestamp,
+        ThreadID,
+        ExecutableName,
         InnermostStackFrame,
         __Count
     };