Summary:
Makes the API more complex, but it's worth it as it allows
knowledgable users to opt into avoding costly work and/or
data copies in very common use cases.

Significantly speeds up a lot of work in the models in the
presence of X apps that can't be identified and fall back to
the window icon.

Based on profiling by David.

Reviewers: dfaure, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D1920