Use folder constants.
Streamline code.

Add missing {}.

Display message when messages are restored.

Rename "Move to inbox" button to "Restore".

Display move to folder drop-down for the inbox too.

#187564: "recycle" implies recovered. Sending a PM to the recycle bin does not really comply to this definition. Delete might not communicate the recycle-ability feature, but is more accurate. Perhaps the confirm_form and the status message can have an additional string that explains that the message can be restored.. But for now, just fixing the terminology will help.

- Remove wasteful preg_match which gets executed on every page for a help tip.
- Remove bit about e-mail integration which used to be part of this module, and is now being taken care of by pm_subscriptions.
- Fix URLs.
- Remove double-spaces.

ca. 1.67

#166609: "missing variable %name in message The recipient %name does not exist." Do not overwrite user input.

Rollback last preview button commit. Remove unnecessary $form initialisation which was causing the issue.