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.