Set icon to wayland if desktop file is not found

Summary:
For X windows KWin uses the generic X icon if no icon is found. Thus
KWin should use the generic Wayland icon if no icon is found for a
Wayland client.

With this change applications with incorrect desktop file name at least
get a generic icon instead of no icon.

Test Plan: Started Qt assistant on Wayland and it had icon.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D3023