Synchconization support

Added support to call a user-specified synchronization command. This allows to use external program to synchronize calendar. For synchronization with Google calendar can be used wget to download file and use downloaded file as Orage external file. Up to 10 synchronization commands allowed. Added button to immediately synchronize all tasks. Only one-way synchronization is supported (from server to Orage).

To set up sync with Google calendar (as May 2023):

1. Enable Google calendar access
2. Add new synchronization settings: Edit->Preferences->Synchronization settings->Add. For synchronization command can be used wget https://calendar.google.com/calendar/ical/<user>/public/<calendar>.ics -O google-cal.ics
3. Add foreign file: File->Exchange Data->Foreign File. New foreign file must have same name as wget downloaded file, in this case google-cal.ics.