LibGUI: Add GVariant class and use it for table model data.

LibGUI/GVariant.h

@@ -0,0 +1,70 @@
+#pragma once
+
+#include <AK/AKString.h>
+#include <SharedGraphics/GraphicsBitmap.h>
+
+class GVariant {
+public:
+    GVariant();
+    GVariant(bool);
+    GVariant(float);
+    GVariant(int);
+    GVariant(const String&);
+    GVariant(const GraphicsBitmap&);
+    ~GVariant();
+
+    enum class Type {
+        Invalid,
+        Bool,
+        Int,
+        Float,
+        String,
+        Bitmap,
+    };
+
+    bool is_valid() const { return m_type != Type::Invalid; }
+    Type type() const { return m_type; }
+
+    bool as_bool() const
+    {
+        ASSERT(type() == Type::Bool);
+        return m_value.as_bool;
+    }
+
+    int as_int() const
+    {
+        ASSERT(type() == Type::Int);
+        return m_value.as_int;
+    }
+
+    float as_float() const
+    {
+        ASSERT(type() == Type::Float);
+        return m_value.as_float;
+    }
+
+    String as_string() const
+    {
+        ASSERT(type() == Type::String);
+        return *m_value.as_string;
+    }
+
+    const GraphicsBitmap& as_bitmap() const
+    {
+        ASSERT(type() == Type::Bitmap);
+        return *m_value.as_bitmap;
+    }
+
+    String to_string() const;
+
+private:
+    union {
+        StringImpl* as_string;
+        GraphicsBitmap* as_bitmap;
+        bool as_bool;
+        int as_int;
+        float as_float;
+    } m_value; 
+
+    Type m_type { Type::Invalid };
+};