libgeda: Add parent PAGE pointer to OBJECT structure.

libgeda: Add parent PAGE pointer to OBJECT structure.

There are numerous examples of libgeda & gschem code that assumes,
when operating on an OBJECT, that it is part of the "current" PAGE as
set in the TOPLEVEL structure.  This has a number of disadvantages,
including the need for regular frobbing of the ADDING_SEL flag and
page_current pointers, but most importantly restricting the ability to
work with more than one PAGE at once. Furthermore, it is not
convenient or practical to add a PAGE argument to all functions that
may eventually need to manipulate or update the current page based on
changes to OBJECTs.

Instead, this patch adds a parent PAGE pointer that is updated *only*
when adding or removing an OBJECT to a PAGE.

In order to ensure consistency in the case of compound objects, only
the "top-level" compound object has its page pointer set; in general,
*either* OBJECT.parent *or* OBJECT.page may be non-NULL.