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/data.h

@@ -376,8 +376,6 @@ struct Window {
 
     /** The name of the window. */
     i3String *name;
-    /** The format with which the window's name should be displayed. */
-    char *title_format;
 
     /** The WM_WINDOW_ROLE of this window (for example, the pidgin buddy window
      * sets "buddy list"). Useful to match specific windows in assignments or
@@ -588,6 +586,9 @@ struct Con {
 
     char *name;
 
+    /** The format with which the window's name should be displayed. */
+    char *title_format;
+
     /* a sticky-group is an identifier which bundles several containers to a
      * group. The contents are shared between all of them, that is they are
      * displayed on whichever of the containers is currently visible */