Gnc-Prefs: stop using gconf as gsettings backend

WARNING: the saved settings in gconf are not (yet) migrated to gsettings !
         running gnucash as of this commit will start you with default values
         for all preferences !

You can of course change your preferences, your changes will be saved
properly in gsettings and restored when you restart gnucash. But what
has been set in GConf before will basically be ignored.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23267 57a11ea4-9604-0410-9ed3-97b8803252fd
Add some debugging code to gnc-gsettings

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23268 57a11ea4-9604-0410-9ed3-97b8803252fd
Test for empty filename in addition to no filename

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23269 57a11ea4-9604-0410-9ed3-97b8803252fd