window: Properly get workspace of transient windows of popups

We set the workspace of popup windows to the same workspace as their transient window, but since the transient window can also be present on all workspaces while the popup should only be on one workspace, we'd set the workspace of the popup to NULL if we use transient_for->workspace directly. Instead, get the workspace of the transient window using meta_window_get_workspace, which returns the current workspace in case the transient window is shown on all workspaces.

This fixes a bug which was exposed by 09bab98b where popup windows opened on secondary monitors would reach the g_assert_not_reached inside meta_window_get_workspaces. That happened because on secondary monitors workspaces are disabled by default, so the workspace of the transient window would be NULL and the workspace of the popup window would get set to NULL.

Fixes #703 (closed)