xwayland/selection: allow unfocused xwayland apps to access clipboard

This restriction did improve security to some degree, but various popular applications don't work properly with it, including wps office and wine. It's impractical for them to finish wayland transition shortly, so relaxing the restriction looks like a more appropriate solution to me.

Bug reports from different wlroots-based wayland compositors: https://github.com/swaywm/sway/issues/6307 https://github.com/hyprwm/Hyprland/issues/2319

ps: I'm unsure if opt-out of the restriction would be better.