GSplitter: Explicitly set the size policies when resizing.

If we don't have the fixed size policy for the first resizee, the layout
system will just ignore the preferred sizes that we're giving it.

Libraries/LibGUI/GSplitter.cpp

@@ -66,7 +66,6 @@ void GSplitter::mousemove_event(GMouseEvent& event)
  // One or both of the resizees were deleted during an ongoing resize, screw this.
  m_resizing = false;
  return;
- ;
  }
  int minimum_size = 0;
  auto new_first_resizee_size = m_first_resizee_start_size;
@@ -88,6 +87,9 @@ void GSplitter::mousemove_event(GMouseEvent& event)
  m_first_resizee->set_preferred_size(new_first_resizee_size);
  m_second_resizee->set_preferred_size(new_second_resizee_size);
 
+ m_first_resizee->set_size_policy(m_orientation, SizePolicy::Fixed);
+ m_second_resizee->set_size_policy(m_orientation, SizePolicy::Fill);
+
  invalidate_layout();
 }