Add bookmarks support.

The library now contains (simple) methods to handle bookmarks.
The bookmark are stored in a separate xml file.

Bookmark are mainly a couple (`zimId`, `articleUrl`).
However, in the xml we store a bit more data :
- The article's title (for display)
- The book's title, lang and date (for potential update of zim files)

include/meson.build

@@ -1,5 +1,6 @@
 headers = [
   'book.h',
+  'bookmark.h',
   'common.h',
   'library.h',
   'manager.h',