Field: Avoid some copy/pasted code.
Document::set_tables(): Check for an actual change.

* glom/libglom/data_structure/tableinfo.h(): Add operator==()
  and operator!=().
* glom/libglom/document/document.cc(): set_tables(): Check for a
  change before re-setting this and marking
  the document as modified.
Document: Rename GLOM_NODE_DATA_LAYOUT_ITEM to GLOM_NODE_DATA_LAYOUT_ITEM_FIELD.