Refactor gnc-tree-view based widgets such that only gnc-tree-view itself deals with gconf

The only exception is gnc-split-reg2, which has it's own implementation
of sort parameters (also linked to gconf unfortunately).

Tree views store column visibility and sort preferences in gconf. This is
actually state information and should be kept in the gnucash state file
(*.gcm in .gnucash/books). It will definitely not be ported to gsettings,
simply because gsettings can't handle floating sections (sections not
strictly tied to schema).

So in the process, where it makes sense the word gconf is already
replaced with "state". That helps with cleaning up the remaining gconf
code in gnucash, and indicates these parts need to be reworked to
use the state file as backend.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23258 57a11ea4-9604-0410-9ed3-97b8803252fd