Store book metadata and nb of pages in new settings

They may already be available (with other statistics stuff) under
the 'stats' key, but not if Statistics plugin is disabled. As this
plugin may be migrated to SQLite, readerui and readerfooter will now
set two new properties: doc_props (containing all and only the metadata
returned by document:getDocumentProps()) and doc_pages (updated
by readerfooter on any display or font change).

PDF document: returns additional PDF properties: Keywords
and Subject (as Description).

frontend/document/pdfdocument.lua

@@ -174,6 +174,8 @@ function PdfDocument:getProps()
     props.authors = props.author
     props.series = ""
     props.language = ""
+    props.keywords = props.keywords
+    props.description = props.subject
     return props
 end