GSettings: add function to bind a gsettings key to a gobject property

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23221 57a11ea4-9604-0410-9ed3-97b8803252fd
Refactor preferences

- rename core-utils/gnc-core-prefs* to core-utils/gnc-prefs*
- rename app-utils/gnc-prefs* to app-utils/gnc-prefs-utils*
- move preferences initialization fuctions to gnc-prefs-utils*
- define a gnc_prefs_* api that is independent of any
  settings backend
- refactor gnc_gsettings_* api to serve as a backend for the
  gnc_prefs_* api

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23222 57a11ea4-9604-0410-9ed3-97b8803252fd