Lua Callbacks

In order of probable implementation.

onFilter

This callback is called for every task in and shows the task (if return is true) or hides it.

onDisplay

Rewrites the task for displaying. This allows filtering of certain parts.

onCalendarSync

Returns the timespan for the calendar appointment or nil if no appointment should be created.

onUpdate

Is called when changing tasks, allows rewriting of the task (for example to sort tags or lists)

onComplete

Maybe to replace the current recurrence implementation. Would be nice if this could call any random intent.

onCreation

Suggested in #349. Is called when creating a task, will provide the source of the task (UI, intent, etc) to provide options to add tags for background tasks.