compose.[ch]: replaced gdk_input_*() with GIOChannel.
added a toggle button for the attachment pane.
reimplemented the attachment list with GtkTreeView. Fixed attaching of locale filename.
added PGP Sign/Encrypt check button on the compose window.
added a menu to temporally change the outgoing encoding to the composition window.
stabilized signature replacement of composition window.
compose.[ch]: removed unrequired member.
Initial import of Sylpheed (GTK2 version).