Give Xed a Sublime-like searchbar

The search dialog is replaced with a searchbar similar to the one used in Sublime.

The following features are implicit and therefore removed:
  - "incremental search"
  - "remove highlights"
  - "parse \n characters"

Code style is applied to impacted files (should have been done in a separate commit but it was being discussed during WIP and the new committed code followed neither the new style nor the old).

xed/xed-window-private.h

@@ -61,6 +61,7 @@ struct _XedWindowPrivate
 
 	/* statusbar and context ids for statusbar messages */
 	GtkWidget      *statusbar;	
+	GtkWidget      *searchbar;	
 	guint           generic_message_cid;
 	guint           tip_message_cid;
 	guint 		tab_width_id;