Rename Painter::set_clip_rect() to add_clip_rect().

It was confusing to see multiple calls to set_foo() in a row. Since this is
an intersecting operation, let's call it add_clip_rect() instead.

LibGUI/GFrame.cpp

@@ -17,7 +17,7 @@ void GFrame::paint_event(GPaintEvent& event)
         return;
 
     GPainter painter(*this);
-    painter.set_clip_rect(event.rect());
+    painter.add_clip_rect(event.rect());
 
     Color top_left_color;
     Color bottom_right_color;