WaylandBackend: re-send app_id and title after the toplevel is unmapped

[gonzalezerik]

Fixes #2446.

When the connector goes invisible, CWaylandPlane::Present() unmaps the toplevel by attaching a NULL buffer. xdg-shell discards all toplevel attributes on unmap ("The xdg_toplevel returns to the state it had right after xdg_surface.get_toplevel"), so the remapped window has no app_id, and no title unless the app's title changed in the meantime, because libdecor_frame_set_title() skips a title it has already sent.

This re-sends both in LibDecor_Frame_Configure() when m_bUnmappedAwaitingConfigure is set, so they go out in the configure that follows the unmap, before the next buffer attach maps the toplevel. That's the spot @nilsenist pointed to in #2446. The title goes through xdg_toplevel_set_title() directly since libdecor would skip it.

Testing, nested in niri (Smithay resets the toplevel on unmap the same way wlroots does), RTX 5070 Ti, Flatpak Steam, Ghost of Tsushima launched with gamescope -W 5120 -H 2160 -r 165 -f --hdr-enabled -- %command%:

Before: after the startup unmap the window came back with no app_id, so a window rule on app_id gamescope stopped applying.
After: the window keeps app_id gamescope and the rule applies.

Built with the Flathub org.freedesktop.Platform.VulkanLayer.gamescope 26.08 manifest on 3.16.31 (current master, 6867f50), together with an unrelated local patch to the HDR metadata path. No new warnings at the project's warning_level=2.

[matte-schwartz]

Since the app_id is only set in the configure callback, the compositor has already handled the initial commit without it, so rules that only apply when a window opens might still miss it. Could you re-send app_id and title right after the unmap instead? The icon gets dropped there too.