Terminal+LibGUI: Make the terminal cursor blink.

Added a GTimer class to help with this. It's just a simple GObject subclass
that sets up an event loop timer and invokes a callback on timeout.

LibGUI/GTimer.h

@@ -0,0 +1,32 @@
+#pragma once
+
+#include <LibGUI/GObject.h>
+#include <AK/Function.h>
+
+class GTimer final : public GObject {
+public:
+    explicit GTimer(GObject* parent = nullptr);
+    virtual ~GTimer() override;
+
+    void start();
+    void start(int interval);
+    void stop();
+
+    bool is_active() const { return m_active; }
+    int interval() const { return m_interval; }
+    void set_interval(int interval) { m_interval = interval; }
+
+    bool is_single_shot() const { return m_single_shot; }
+    void set_single_shot(bool single_shot) { m_single_shot = single_shot; }
+
+    Function<void()> on_timeout;
+
+    virtual const char* class_name() const override { return "GTimer"; }
+
+private:
+    virtual void timer_event(GTimerEvent&) override;
+
+    bool m_active { false };
+    bool m_single_shot { false };
+    int m_interval { 0 };
+};