show chapters in bookmark dialog

This is a stripped down port of @notspiff chapter menu. Removed:
- Thumbs. It's too buggy and extremely slow in its current state. 
- Chapter names. On every file/dvd I could find, chapter name was the the of the chapter (i.e it shows two timestamps in GUI). Changed it to using generic "Chapter i" until a better solution can be found.

I'd prefer this to be done properly in a new window/menu as a simple list, but until somebody does that, this is a working alternative.