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/NetworkStatisticsWidget.h

@@ -1,11 +1,11 @@
 #pragma once
 
 #include <LibCore/CTimer.h>
-#include <LibGUI/GWidget.h>
+#include <LibGUI/GLazyWidget.h>
 
 class GTableView;
 
-class NetworkStatisticsWidget final : public GWidget {
+class NetworkStatisticsWidget final : public GLazyWidget {
     C_OBJECT(NetworkStatisticsWidget)
 public:
     virtual ~NetworkStatisticsWidget() override;