Summary:
Windows we can't find an app icon for using the normal means
get the icon used by the windowing system in the Task Manager.
This fallback icon was then not updated when changed on the
window, only occasionally as a side-effect of cache evictions
and model data requests.

This patch notes which windows hit the fallback path, and for
those windows evicts the cache when the icon changes on the
window, causing it to be re-retrieved from the windowing system
as views respond to the dataChanged signal for the decoration
role.

Evicting the entire cache is a little bit costly, but:
- This is a fallback codepath.
- Apps conventionally update title and icon in one go, meaning
  the cache is often already getting evicted anyway.
- Icons don't change that often.

BUG:383017

Reviewers: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7092