Mass api change from GtkExtendedLayout --> GtkSizeRequest

Mass api change from GtkExtendedLayout --> GtkSizeRequest

This commit makes a few massive changes to the extended layout
code:
  a.) gtkextendedlayout.c --> gtksizerequest.c
  b.) _is_height_for_width --> get_request_mode()
  c.) get_desired_size(), get_desired_width(), get_desired_height() -->
      get_size(), get_width(), get_height()

This is the first partial commit and only effects portions
of the tree that have already been merged in master (in order to
easily cherry pick this commit).

Conflicts:

	gtk/Makefile.am
	gtk/gtk.h
	gtk/gtk.symbols
	gtk/gtkextendedlayout.h