Add support for export/import of external program tasks

[mountaindude]

Both in terms of including such tasks in export/import files, but also as part of reload chains (i.e. in composite triggers).

[mountaindude]

This change require updates to these commands:

• task-import (same parameters as before, but ext-program should now be a valid task type in the task definitions file)
  • Included in v3.14
• task-custom-property-set (new task type parameter ext-program that can be used to filter out external program tasks)
• task-get (new task type parameter ext-program that can be used to filter out external program tasks) when showing task tables.
  • Included in v3.14

[mountaindude]

Implemented in v3.10 for task-get command.

[mountaindude]

See #323.
The task-import command fails if there are external program tasks in the task definitions file.

[mountaindude]

Version 3.14 was just released (download here), it contains (among other things) changes related to this ticket.