Rename gnc-main to gnc-core-prefs

And add proper namespace prefixes to its functions

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@22939 57a11ea4-9604-0410-9ed3-97b8803252fd
Add new core prefs

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@22940 57a11ea4-9604-0410-9ed3-97b8803252fd