ZODB Components: Fix deadlock happening when generating a new UID while installing bt5 (7d31f4f).

7d31f4f allowed to prevent duplication of Component, as the ID was not enough,
for example when renaming a Component. As getUid() may trigger generation of a
new UID (and thus relying on portal_ids) and is protected by a non-reentrant
lock, it may deadlock when generating Component registry_dict while loading
Portal Types.
ZODB Components: Cosmetic: Use faster and simpler defaultdict() rather than setdefault().
Support properly HTML5 implicit form submission.

From Chromium 31, pressing enter key to select ListBox page submit the form
through JS (submitFormOnEnter) *and* wrongly on the default button, at the end
following HTML5 behavior (4.10.22.2 Implicit submission).