Add convenience method to get a persistent model index from TasksModel

Authored by davidedmundson on Mar 5 2018, 2:17 PM.

Description

Add convenience method to get a persistent model index from TasksModel

Summary: Used in the minimizeAll plasmoid (see D10019)

Test Plan: Used in applet

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D11059