LibGUI: Fix bad initial layout of GScrollableWidget's scrollbars.

If it's the first time we're laying these out, we can't rely on the width()
or height() to be set, so instead use preferred_size().

LibGUI/GScrollableWidget.cpp

@@ -30,8 +30,8 @@ void GScrollableWidget::resize_event(GResizeEvent& event)
     auto inner_rect = frame_inner_rect_for_size(event.size());
     update_scrollbar_ranges();
 
-    int height_wanted_by_horizontal_scrollbar = m_horizontal_scrollbar->is_visible() ? m_horizontal_scrollbar->height() : 0;
-    int width_wanted_by_vertical_scrollbar = m_vertical_scrollbar->is_visible() ? m_vertical_scrollbar->width() : 0;
+    int height_wanted_by_horizontal_scrollbar = m_horizontal_scrollbar->is_visible() ? m_horizontal_scrollbar->preferred_size().height() : 0;
+    int width_wanted_by_vertical_scrollbar = m_vertical_scrollbar->is_visible() ? m_vertical_scrollbar->preferred_size().width() : 0;
 
     m_vertical_scrollbar->set_relative_rect(inner_rect.right() + 1 - m_vertical_scrollbar->preferred_size().width(), inner_rect.top(), m_vertical_scrollbar->preferred_size().width(), inner_rect.height() - height_wanted_by_horizontal_scrollbar);
     m_horizontal_scrollbar->set_relative_rect(inner_rect.left(), inner_rect.bottom() + 1 - m_horizontal_scrollbar->preferred_size().height(), inner_rect.width() - m_vertical_scrollbar->preferred_size().width(), width_wanted_by_vertical_scrollbar);