macosx: fix and clean-up the bookmarks code (close #6947)

The code is far from being great, but it matches the Qt intf now.
(cherry picked from commit cd612818db02b7831bec1d72e0a864f968daed6a)

Conflicts:
	modules/gui/macosx/bookmarks.m

modules/gui/macosx/bookmarks.h

@@ -49,6 +49,7 @@
 
     input_thread_t *p_old_input;
 }
++ (VLCBookmarks *)sharedInstance;
 
 - (IBAction)add:(id)sender;
 - (IBAction)clear:(id)sender;
@@ -60,9 +61,6 @@
 - (IBAction)edit_cancel:(id)sender;
 - (IBAction)edit_ok:(id)sender;
 
-+ (VLCBookmarks *)sharedInstance;
-
-- (void)initStrings;
 - (void)showBookmarks;
 - (id)dataTable;
 @end