Add a user_symbol to one UT

Author:    Frédéric Perrin <frederic.perrin@resel.fr>

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23324 57a11ea4-9604-0410-9ed3-97b8803252fd
Add a field to the commodity editor allowing setting the user symbol.

Author:    Frédéric Perrin <frederic.perrin@resel.fr>

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23325 57a11ea4-9604-0410-9ed3-97b8803252fd