ProcessManager: Add CPU/memory usage graphs in a separate tab.

Finally we get some real use for the new GTabWidget. :^)

Applications/ProcessManager/GraphWidget.h

@@ -0,0 +1,24 @@
+#include <LibGUI/GFrame.h>
+#include <AK/CircularQueue.h>
+
+class GraphWidget final : public GFrame {
+public:
+    explicit GraphWidget(GWidget* parent);
+    virtual ~GraphWidget() override;
+
+    void set_max(int max) { m_max = max; }
+    void add_value(int);
+
+    void set_graph_color(Color color) { m_graph_color = color; }
+    void set_text_color(Color color) { m_text_color = color; }
+
+    Function<String(int value, int max)> text_formatter;
+
+private:
+    virtual void paint_event(GPaintEvent&) override;
+
+    int m_max { 100 };
+    CircularQueue<int, 4000> m_values;
+    Color m_graph_color;
+    Color m_text_color;
+};