Revised method allowMessageToBeQueued()

Also made $limit a private property of the class. This limits the total
size of all the inline images together.
Initialization & constructor now work in all cases.

Removed pages now redundant. The plugin now holds only temporary image
files.
Everything works up to the messageQueued method

Have changed the principle regex. I have tested it, but we should still
check that the allowMessageToBeQueued method still works as correctly
as before.
All parts tested, at least a bit. This version appears to work.
Handle email forwards also

Earlier versions could not include inline image attachments in emails
forwarded through phplist. We have corrected this problem.
Just cleaned up the file a little bit
Better handle re-queued and forwarded messages

Prevented duplication of entries in message table for re-queued and
forwarded messages. Handled forwarded messages even if image files have
been deleted from the plugins image subdirectory. We’ve also set the
image subdirectory to be cleaned out when it contains more than 2 MB
worth of files.
Merge branch 'Simpler-version'