workspaceAnimation: Assign movingWindow before creating window actors

workspaceAnimation: Assign movingWindow before creating window actors

The BaseWorkspaceGroup constructor calls _createWindows(), which uses
_shouldShowWindow() to determine which actors to create for the
animation.

In WorkspaceGroup _shouldShowWindow() this depends on a comparison with
this._movingWindow, which however only gets set after the
BaseWorkspaceGroup constructor has been called.

This was causing the moving window to no longer stick to the screen
when moved between workspaces.

Fixes: 96be47933 ("workspaceAnimation: Split out BaseWorkspaceGroup")
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8992
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4092>