Messages in threads with no parent count as separate threads in mailbox list limit

[trisk]

Some threads can have many messages referring to a parent which is not present (an example is the emails from this Trac instance, if I am CC'ed on a ticket I did not create). These threads cannot be expanded/unexpanded, as discussed in #1486571 and RFC 5256 says they should have a dummy message as their parent. The messages still count as separate threads or rows towards the mailbox list limit (default 40) while threads with a single parent only count the parent. This is a problem if the thread is particularly long as it can take up more than the thread limit and keep all other messages on previous pages.

Counting them as a single thread for the purposes of the mailbox list is a possible solution, but that could make the mailbox view a display lot more rows than is reasonable.

[trisk]

As a solution, I propose that parent-less threads should still allow (un)expansion using a dummy parent message, displaying the most recent message when not expanded. I'll investigate the implementation when I have time.