Add an inline grouping mode.

Summary:
When grouping inline, the group subtrees are flattened out into
the top-level list, parents removed, and move() treats groups as
singular entities.

This functionality was previously found in Netrunner's Expanding
Icons Task Manager fork.

This is the first half of the EITM functionality merge.

Reviewers: davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D1881