LibGUI: Add a simple GSplitter container widget.

This allows you to put multiple widgets in a container and makes the space
in between them draggable to resize the two adjacent widgets.

SharedGraphics/Point.h

@@ -49,6 +49,7 @@ public:
     }
 
     Point operator-() const { return { -m_x, -m_y }; }
+    Point operator-(const Point& other) const { return { m_x - other.m_x, m_y - other.m_y }; }
 
     operator WSAPI_Point() const;
     String to_string() const { return String::format("[%d,%d]", x(), y()); }