LibGUI: Support bitmaps in GTableView cells.

Use this to add an icon for each process in the ProcessManager.
Right now they all use a generic gear icon, but I'd like to have
per-process icons, obviously. :^)

Applications/ProcessManager/ProcessTableModel.h

@@ -44,4 +44,5 @@ private:
     HashMap<pid_t, OwnPtr<Process>> m_processes;
     Vector<pid_t> m_pids;
     int m_selected_row { -1 };
+    RetainPtr<GraphicsBitmap> m_generic_process_icon;
 };