IntervalPlugin should not depend on scheduled event

[rsoika]

Currently the IntervalPlugin did not run if the event is not a scheduled event.
This restriction does not make sense as it only reduces the functionality of this plugin

The goal is to run the IntervalPlugin if a item 'interval' was defined in the workflow.

Example:

<item name="interval">
    <ref>contract.reminder</ref>
    <macro>@monthly</macro>
</item>