Move title_format from window to container.

This patch moves the title_format information from windows to containers.
Furthermore, it allows correctly setting it on window-less containers and
displays the title accordingly for split containers.

We now also dump and read title_format in GET_TREE / during restarts.

fixes #2120

include/window.h

@@ -81,10 +81,3 @@ void window_update_hints(i3Window *win, xcb_get_property_reply_t *prop, bool *ur
  *
  */
 void window_update_motif_hints(i3Window *win, xcb_get_property_reply_t *prop, border_style_t *motif_border_style);
-
-/**
- * Returns the window title considering the current title format.
- * If no format is set, this will simply return the window's name.
- *
- */
-i3String *window_parse_title_format(i3Window *win);