steamcompmgr: Advertise UTF8_STRING in clipboard TARGETS.

[ivyl]

Software respecting ICCCM will ask for TARGETS first before asking for one of the supported formats. Advertising only STRING will break with any non-ANSI characters, especially since handle_selection_notify() process UTF8 only and gamescope only returns UTF8 strings.

This fixes non-ANSI text copied from Firefox / Chrome running on a Steam deck into any Wine app being a garbled mess.

[ivyl]

I had a look at using https://gitlab.freedesktop.org/wlroots/wlroots/-/tree/master/xwayland/selection but it's deeply integrated into their xwm. Best we could do is just use it as an inspiration for a better gamescope implementation.

I think that properly advertising UTF8 is good enough for now though. I haven't found anything that would not advertise or respect UTF8_STRING TARGETS.