Store normal clients and desktop clients in the same list

Summary:
On X11, Workspace stores windows in two lists. One with desktop windows
and the other one with all other windows. On Wayland, desktop windows
and normal windows are stored in the same list - m_allClients.

In order to unify scripting on X11 and Wayland, this change makes the
Workspace class store X11 desktop windows and normal X11 windows in the
same list. It's the responsibility of scripts to filter desktop windows.

Reviewers: #kwin, apol

Reviewed By: apol

Subscribers: apol, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29522