Profiler: Split the call tree into one subtree per process

This patch adds an additional level of hierarchy to the call tree:
Every process gets its own top-level node. :^)

Before this, selecting multiple processes would get quite confusing
as all the call stacks from different processes were combined together
into one big tree.

Userland/DevTools/Profiler/Process.h

@@ -54,6 +54,7 @@ struct Thread {
 struct Process {
     pid_t pid {};
     String executable;
+    String basename;
     HashMap<int, Vector<Thread>> threads {};
     LibraryMetadata library_metadata {};
     u64 start_valid;