SystemMonitor: Make all tabs except the process table lazily loaded

We now use GLazyWidget for all the secondary tabs, which makes the
program start up way faster than before.

There's a noticeable delay when you click on the "PCI Devices" tab
for the first time, but that's definitely better than always eating
that delay before seeing a window at all. :^)

Applications/SystemMonitor/ProcessTableView.h

@@ -19,5 +19,5 @@ public:
     Function<void(pid_t)> on_process_selected;
 
 private:
-    ProcessTableView(GraphWidget&, GWidget* parent);
+    explicit ProcessTableView(GWidget* parent = nullptr);
 };