Summary:
KWindowSystem provides a KDE specific property for the desktop file
name. This allows KWin to take the icon from the desktop file. The
advantage from the desktop file is that KWin normally gets higher
resolution icons than provided through the xproperty based icons used
previously. If the desktop file does not provide an icon name, KWin
falls back to the previous implementation.

As on Wayland the icon is taken from the desktop file name already the
code for X11 and Wayland is merged in AbstractClient. Also to the
PlasmaWindowInterface the appId is taken from the new desktop file
instead of the resourceName. Due to that for Xwayland windows where KWin
knows the desktop file name it can be passed to PlasmaWindowInterface.
This allows e.g. the task manager to better map the windows to
applications and provide better icons. Also it means that icons do not
need to be passed as bitmap data to the clients.

Test Plan:
Verified that icon is taking from desktop file if provided and
from X property if not provided and that Wayland windows still have icon.

Reviewers: #kwin, #plasma_on_wayland, hein

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3177