LibGUI: Improve GBoxLayout so it can better support GToolBar.

Added spacing and margin concepts to GLayout. Support layout a sequence
of nothing but fixed-size objects in the desired orientation. :^)

LibGUI/GMargins.h

@@ -0,0 +1,40 @@
+#pragma once
+
+class GMargins {
+public:
+    GMargins() { }
+    GMargins(int left, int top, int right, int bottom)
+        : m_left(left)
+        , m_top(top)
+        , m_right(right)
+        , m_bottom(bottom)
+    {
+    }
+    ~GMargins() { }
+
+    bool is_null() const { return !m_left && !m_top && !m_right && !m_bottom; }
+
+    int left() const { return m_left; }
+    int top() const { return m_top; }
+    int right() const { return m_right; }
+    int bottom() const { return m_bottom; }
+
+    void set_left(int value) { m_left = value; }
+    void set_top(int value) { m_top = value; }
+    void set_right(int value) { m_right = value; }
+    void set_bottom(int value) { m_bottom = value; }
+
+    bool operator==(const GMargins& other) const
+    {
+        return m_left == other.m_left
+            && m_top == other.m_top
+            && m_right == other.m_right
+            && m_bottom == other.m_bottom;
+    }
+
+private:
+    int m_left { 0 };
+    int m_top { 0 };
+    int m_right { 0 };
+    int m_bottom { 0 };
+};