Fix message filtering on startup.

When the FilterManager was constructed it tried to filter the
unread messages in the inbox before it had loaded the filtersettings.
This caused it only to filter new messages while it was running.

With this patch the tryToFilterInboxOnStartup function will always
be called when new filters are added. (e.g. After the config is read)
The delay in which the filter checks if the inbox is valid has also
been reduced to avoid situations where the user can look at a mail
in the inbox which is about to be filtered.

This patch was provided by steveire