SystemMonitor: Display processes and their threads in a tree :^)

This shows all non-main threads as children of the process they belong
to. We also show the TID as that is important to distinguish the
different threads in one process.

Fixes #65

:skeleyak:

Userland/Applications/SystemMonitor/SystemMonitor.gml

@@ -23,9 +23,11 @@
                     spacing: 0
                 }
 
-                @GUI::TableView {
+                @GUI::TreeView {
                     name: "process_table"
                     column_headers_visible: true
+                    should_fill_selected_rows: true
+                    selection_behavior: "SelectRows"
                 }
             }