LibGUI: Take ProcessManager's process view and turn it into GTableView.

Make it sufficiently generic that it can be reused for any table data. :^)

Applications/ProcessManager/ProcessTableModel.cpp

@@ -141,6 +141,8 @@ void ProcessTableModel::update()
     }
     for (auto pid : pids_to_remove)
         m_processes.remove(pid);
+
+    did_update();
 }
 
 pid_t ProcessTableModel::selected_pid() const