This caused two problems:
 - If Xfvb was killed and left a stale lock file, we don't reuse this
   display. After some time we end up having "All display locked"
   exception but no Xvfb process running.
 - Xvfb's slapos is patched to store control sockets and lockfiles in
   $TMPDIR ( nexedi/slapos@58462c24 ).
   We don't have $TMPDIR set in testnodes, but this would be a problem
   if we ever set it.

Because Xvfb can detect a stale lock file upon startup and start anyway,
the easy way running Xvfb for each display and see if it's still running
after a few seconds.

Models with workflow state "default" are only considered in composition if no
other valid model is defined. This allows to implement "ingest anything"
policy in wendelin.

Add PortalType field to distinguish different types of Python Scripts, add referencence and use source code editor

zope.{interface,component} implement Interfaces through __implements__,
__implemented__ (both implementedBy instances) and __provides__ (ClassProvides
instance) attributes set on the class itself through implementedByFallback
(zope.interface.declarations).

However, this implementation creates circular references by referencing the
class itself and thus erp5.* classes (and all its Accessors instances) were
never GC even after a reset.

When running testXHTML and installing the Unit Tests bt5s:
  * After 10 resets:  21MB leak (~  7% of Zope process USS).
  * After 20 resets:  70MB leak (~ 18% of Zope process USS).
  * After 28 resets: 122MB leak (~ 26% of Zope process USS).

Fixes browsing ERP5 documents without activity tables available.
Of course, as most modifications trigger activity creation, no
modifications are possible. But this improves read-only access in
degraded conditions.
Also, by design a successful query must return one line, so do not test
returned value before processing it.

Allows recovering from broken connection strings, as ERP5 authentication
still relies on having a working catalog.

erp5_travel_expense: Record creation is eventless

erp5_travel_expense: Representative Record creation is done by an alarm