fix: activate stacked windows without redundant raising

[philip-sterne]

Summary

• Show and focus a stacked window before updating Pop Shell's active-tab state.
• Let Stack.activate() own tab visibility and styling updates.
• Remove the redundant Meta.Window.raise() after activate_with_focus().

Root cause

The tab click handler changed Pop Shell's active-window state and hid the previous actor before confirming that the selected window actor could be activated. It then explicitly raised a window that Mutter had already focused. This could leave compositor stacking and Pop Shell's stack state temporarily out of sync, producing meta_window_set_stack_position_no_sync assertions and unreliable tab activation.

Impact

Stack tab clicks now follow one activation sequence and avoid the redundant stacking operation.

This is independent of #1824, which prevents DING's desktop surface from entering the normal window/focus path. It was field-tested in a Wayland session that also contained #1824.

Testing

• make compile
• Logged out and back in on Ubuntu with GNOME 50.
• Repeatedly activated windows through Pop Shell stack tabs during normal use.