remove a bunch of explicit uses of '/' as a directory separator; use Glib::build_file...

remove a bunch of explicit uses of '/' as a directory separator; use Glib::build_filename() to construct more paths rather than doing it "by hand"; fixup small logic mistake (which had no actual consequences) in export track marker dialog

git-svn-id: http://subversion.ardour.org/svn/ardour2/ardour2/branches/2.0-ongoing@6909 d708f5d6-7413-0410-9779-e7cbd77b26cf