LibGUI: Allow widgets to clip their child widgets

This patch adds Widget::children_clip_rect() which can be overridden
to tighten clipping of a widget's children. The default implementation
simply returns Widget::rect().

Libraries/LibGUI/Widget.h

@@ -274,6 +274,8 @@ public:
     void set_accepts_emoji_input(bool b) { m_accepts_emoji_input = b; }
     bool accepts_emoji_input() const { return m_accepts_emoji_input; }
 
+    virtual Gfx::IntRect children_clip_rect() const;
+
 protected:
     Widget();