class_docstr: Define an application like layout of widgets.
  
  Parameters
  ----------
  
  header: instance of Widget
  left_sidebar: instance of Widget
  center: instance of Widget
  right_sidebar: instance of Widget
  footer: instance of Widget
      widgets to fill the positions in the layout
  
  merge: bool
      flag to say whether the empty positions should be automatically merged
  
  pane_widths: list of numbers/strings
      the fraction of the total layout width each of the central panes should occupy
      (left_sidebar,
      center, right_sidebar)
  
  pane_heights: list of numbers/strings
      the fraction of the width the vertical space that the panes should occupy
       (left_sidebar, center, right_sidebar)
  
  {style_params}
  
  Examples
  --------
code: def _convert_sizes(self, size_list):
      return list(map(self._size_to_css, size_list))
  
imports: import re
  from collections import defaultdict
  from traitlets import Instance, Bool, Unicode, CUnicode, CaselessStrEnum, Tuple
  from traitlets import Integer
  from traitlets import HasTraits, TraitError
  from traitlets import observe, validate
  from .widget import Widget
  from .widget_box import GridBox
  from .docutils import doc_subst
label_desc: Use image node_id 3 for calling the AppLayout obj's underlying member method code with example usage: obj._convert_sizes(size_list) and returns: list