entirely delete obsolete item modification code
item description: item list

Use the Stdlib class instead of direct DB access.
Also change the generated output.
update STDLIB_PENDING table

* use int for `update` instead of enum (UpdateType class constants)
* add a `comment` attribute
* make `lib` the PRIMARY KEY
rename StdlibPending method
move get_update function into UpdateType class
fix relative require_once() calls in StdlibRelease.php
move logic for finding pending entries for a release

Move the code from update.php to the StdlibPending class. Fix,
refine and shorten it.
support release description item list for unpublished releases

Also slightly modify output.
Stdlib::GetItems() - support unpublished releases

Check if a release is unpublished. If so, get the latest
published release and filter them by the pending changes
for the given release. Adjust the code describing stdlib
releases.
modify StdlibPending::GetAllEntries() output
support changelog for unpublished releases
remove update type from release description