LibWeb: Position the ListItemMarkers according to their width

Previously they were positioned with a fixed offset. However this lead
to wider markers with more than one character to collide with the
element itself.
Now the ListItemMarkerBox generates and stores the appropriate String
in its constructor and sets its own width according to that.
The ListItemBox then lays out the Marker taking this width into
account.
This also made the painting a lot easier since we don't generate the
needed Strings every time we repaint, just once.

Userland/Libraries/LibWeb/Layout/ListItemBox.cpp

@@ -37,8 +37,8 @@ void ListItemBox::layout_marker()
         append_child(*m_marker);
     }
 
-    m_marker->set_offset(-8, 0);
-    m_marker->set_size(4, line_height());
+    m_marker->set_offset(-(m_marker->width() + 4), 0);
+    m_marker->set_height(line_height());
 }
 
 }