Give only focus to child popups on lxqt-panel

[stefonarch]

See https://github.com/lxqt/lxqt-panel/blob/a9838b3b2ba5492a19b82044a28b50b12dcf2650/panel/lxqtpanel.cpp#L262

In Kwin, wayfire and Hyprland the panel doesn't steal focus if clicked or when focus-on-mouseover is used - it would be nice to have that also in Niri.

[YaLTeR]

So uh, we intentionally stopped giving keyboard focus to child popups of non-keyboard-interactive layer surfaces because it breaks GTK 3 bars: #311 #818

[stefonarch]

I see, it looks like they are mouse-only (tested waybar and sfwbar) - no menu navigation by keyboard in popups and also no menu search in applications menus (if any exists).

Instead xfce4-panel has exactly the same issue as lxqt-panel as both use Keyboard interactivity: on-demand.