* Update the data collection format & handler to make it much less tortuous
* Update the pagecount & resync the stats on document layout changes
* Update the database schema to allow doing most queries against a SQL view that rescales the collected data to be accurate regardless of document layout (thanks to @marek-g for the SQL magic ;)).
* Add a "reset stats for current book" entry in the list of reset options, one that won't horribly break stats in said book ;).
* Fixed a couple of resource (SQL connection) leaks (in ReaderStatistics:getCurrentBookStats & ReaderStatistics:getCurrentBookStats).
* Flush stats to the DB on periodical metadata saves.
* Minor cosmetic tweaks to the code
Remove the subprocess management functions from xutil.lua, as they were
moved into base/ffi/util.lua, and use them from there.
Also use the cre_storage_size_factor setting when processing credocuments,
to avoid CRE WARNING while indexing too.
Fix dealing with MuPDF document opening failures (previously, these
were not noticed and indexing was retried each time).
* CoverBrowser plugin: alt views for File Browser and History
* Added Prune cache and Compact cache menu actions
* Support for book descriptions, and settings stored in db
