Rewrite IMAP flags handling/display code

Rewrite IMAP flags handling/display code

Rewrite to match recent search rewrite. Biggest benefit: removes a large
amount of logic from the preferences file (and, thus, wasted storage in
the session).

Move all flags to separate objects, and use these objects to interact
with the various flag quirks.

This commit does make a fairly significant UI change: you no longer mark
a message as 'Unseen', you unmark it as 'Seen'.  Not quite as intuitive
to me, but this is the way the IMAP specs define it and the way that
other MUAs (e.g. Thunderbird) show to user. But at the internal level it
allows us to get rid of the 'inverse' flagging.

Fix bug: Allow filtering of ALL IMAP flags in dimp, not just user settable
ones.

fullmessage-dimp.js -> message-dimp.js

For those upgrading that have custom flags you don't want to lose, this
script is what I used to convert.  YMMV.

<?php
require_once dirname(__FILE__) . '/lib/Application.php';
Horde_Registry::appInit('imp', array('authentication' => 'none'));

$registry->setAuth(***USERNAME***, array());

$msgflags = array_merge(
    json_decode($prefs->getValue('msgflags_user'), true),
    json_decode($prefs->getValue('msgflags'), true)
);
$out = array();

foreach ($msgflags as $key => $val) {
    if ($val['t'] == 'imapp') {
        $out[] = new IMP_Flag_User($val['l'], $key, $val['b']);
    }
}

$prefs->setValue('msgflags', serialize($out));
$prefs->remove('msgflags_user');