workspace: Add meta_workspace_get_default_focus_window()

Each workspace has a window that will be focused when switching to that
workspace. Add a function to retrieve that window.

This is only relevant for click-to-focus focus mode, since with the two
other modes no window will be focused upon switching, and will only gain
focus when hovered.

This will be used in the next commit to make this window appear focused.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/850>

src/core/workspace-private.h

@@ -92,6 +92,7 @@ GList* meta_workspace_get_onscreen_region       (MetaWorkspace *workspace);
 GList * meta_workspace_get_onmonitor_region (MetaWorkspace      *workspace,
                                              MetaLogicalMonitor *logical_monitor);
 
+MetaWindow * meta_workspace_get_default_focus_window (MetaWorkspace *workspace);
 void meta_workspace_focus_default_window (MetaWorkspace *workspace,
                                           MetaWindow    *not_this_one,
                                           guint32        timestamp);