Make Workspace::allClientList() return all windows

Currently windows are scattered in a few separate lists. If you need to
go through the windows, you have to do it piece by piece. On the other
hand, with the overhaul of window types, we've started converging
towards one universal type: Window. Keeping windows in the separate
buckets goes against this design.

Workspace::stackingOrder() already contains all windows. This change
repurposes Workspace::allClientList() from a list of "normal" windows to
all windows, i.e. Workspace::windows(), to be consistent.

There's one API change though. Scripting API will expose other window
types too. This is an intentional change so scripted effects could
operate with all windows. It also matches the current behavior observed
in libkwineffects, which exposes all windows as well.