synchronize buffer list changes (add/close/merge) if only syncing open buffers.
requires weechat 1.1, but will not crash anything if weechat < 1.1 is used. fixes #150  #159 #167
make sure we process buffers which don't have last read line or highlight data just like the others
synchronize hotlist every 5 minutes in both modes
adjusting the hot messages that are displayed in the notification as needed
change bell icon accordingly to only sync open buffers setting
synchronize hotlist every 5 minutes. somewhat fixes #167 #166
minor improvements as suggested by @mhoran
renamed the receiver class and adjusted preference text
don't use peekService() as it doesn't work reliably
the docs don't mention it but peekService returns null if the service has no binder
that can happen when the service is restarted by the system after OOM kill