ProcessManager: Show some basic system memory stats below the process table.

This really improves the feeling of "system overview" :^)

Applications/ProcessManager/MemoryStatsWidget.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <LibGUI/GWidget.h>
+
+class GLabel;
+
+class MemoryStatsWidget final : public GWidget {
+public:
+    explicit MemoryStatsWidget(GWidget* parent);
+    virtual ~MemoryStatsWidget() override;
+
+    void refresh();
+
+private:
+    virtual void timer_event(GTimerEvent&) override;
+    virtual void paint_event(GPaintEvent&) override;
+
+    GLabel* m_user_physical_pages_label { nullptr };
+    GLabel* m_supervisor_physical_pages_label { nullptr };
+    GLabel* m_kmalloc_label { nullptr };
+};