bookmarks: Don't use g_strconcat() to build paths
bookmarks: Don't use g_strconcat() to build paths

Use g_build_filename() instead.

https://bugzilla.gnome.org/show_bug.cgi?id=673912

MathieuDuponchelle/grilo-plugins,grilofw/grilo-plugins,vrutkovs/grilo-plugins,jasuarez/grilo-plugins,GNOME/grilo-plugins,GNOME/grilo-plugins,vrutkovs/grilo-plugins,MathieuDuponchelle/grilo-plugins,vrutkovs/grilo-plugins,grilofw/grilo-plugins,MikePetullo/grilo-plugins,MathieuDuponchelle/grilo-plugins,vrutkovs/grilo-plugins,MathieuDuponchelle/grilo-plugins,MikePetullo/grilo-plugins,MikePetullo/grilo-plugins,jasuarez/grilo-plugins