Parse the title_format and display the customized window title if a format was set.

The format string set with "title_format" can contain the placeholder "%title" which will be replaced with the actual window title.

By not overwriting window->name itself, we make sure that assignment matching still works as expected.

fixes #1723

include/data.h

@@ -363,6 +363,8 @@ 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