* glom/libglom/data_structure/print_layout.[h|cc]: Add get/set_page_count().
* glom/libglom/document/document.cc: load_after(), save_before():
Store the page count in the document.
* glom/glom_document.dtd: Mention the new attribute.

* glom/print_layout/canvas_print_layout.[h|cc]: Add get/set_page_count()
and get_page_bounds().
update_page_bounds(): Move the margin creation into here, and create
margins for all pages.
* glom/utility_widgets/canvas/canvas_group_grid.[h|cc]:
  Added update_grid_for_new_size().
* glom/mode_design/print_layouts/window_print_layout_edit.[h|cc]: Add
Add Page and Delete Page menu items, though we need to rearrange the menus
more sensibly.

* glom/printoperation_printlayout.cc: on_begin_print(), on_paginate():
  Set the number of pages.
  on_draw(): Render only the specified page's bounds from the GooCanvas.
  However, this is not working for any page but the first one.
  Some cairo transformation might be necessary.