ProcessList_buildTree: sort by parent for fast search

ProcessList_buildTreeBranch used to search for children with a linear
scan of the process table, which made tree build time quadratic in
process count. Pre-sorting the list by parent PID (if known) makes it
possible to select the correct slice by bisection much faster.

Process.h

@@ -255,6 +255,9 @@ typedef struct Process_ {
    unsigned int tree_depth;
    unsigned int tree_index;
 
+   /* Has no known parent process */
+   bool isRoot;
+
    /*
     * Internal state for merged Command display
     */