The WPrefs menu editor creates standalone draggable windows for each
menu item via W_CreateTopView. On X11 these are override-redirect
windows. On Wayland they were becoming framed XDG toplevels with
taskbar entries, cluttering the desktop with phantom 'WINGs...' tabs.
Implementation:
1. New W_View flags.popup field, set by editmenu's standalone mode
2. wclient_wl: when realizing a popup view, set xdg_toplevel app_id
to 'wings-popup' as a hint to the compositor
3. wmaker compositor: detect 'wings-popup' app_id in toplevel_map,
set borderless + skip-taskbar + skip-switchpanel flags, then call
wWindowConfigureBorders to rebuild the frame without decorations
4. New W_EventBackend->window_begin_move vtable method:
- X11: returns False (caller runs manual drag loop)
- Wayland: calls xdg_toplevel_move, returns True (compositor drives drag)
5. editmenu's dragMenu calls window_begin_move first; on Wayland the
compositor handles the entire drag, so the manual loop is skipped
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