So an image file can have a cover in coverbrowser's display.
It also allows for an alternative viewer (ImageViewer widget)
when holding on file / View full size cover.
- Factored out duplicate code from filemanager.lua and filemanagerhistory.lua
to new filemanagerbookinfo.lua (and other common code to filemanagerutil.lua).
- Uses sidecar files' new doc_props and doc_pages settings, or fallback to
old 'stats' settings, or to opening document.
- Shows filename, filetype and directory.
- Shows description (Hold to see whole truncated text), keywords, and
cover image (tap to extract image from document and display it if available).
- Book information now available from reader menu, to display info about
the currently opened book.
- Convert possibly HTML description to plain text via added
util.htmlToPlainTextIfHtml() (for simple HTML conversion).
