Support passing generic QIcon through PlasmaWindow interface

Summary:
Especially for Xwayland windows the compositor might not have a themed
icon name. Resulting in a task manager not having dedicated icons for
Xwayland windows.

This change deprecates the way how a compositor is supposed to set the
window icon. Instead of passing the themed icon name, it is now supposed to
pass the QIcon. In case it's a themed icon the existing way to pass to
the client is used.

Otherwise a new event is used to inform the client that there is an icon
- no data is transmitted at this point. The client can then create a
file descriptor and pass it to the compositor. The compositor serializes
the icon into the file descriptor and the client can read from it. This
all happens transparently on client side there is no api change at all.

The writing and reading of the icon is done in a thread. Due to that
Qt5::Concurrent is now a required dependency instead of an optional
dependency.

Reviewers: #plasma_on_wayland, hein

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D3049