WaylandBackend: paste the host clipboard into nested XWayland

[craimasjien]

My first attempt adding functionality/contributing to gamescope, bear with me.

This PR adds the missing host → guest clipboard path to the nested Wayland backend, so the host session's clipboard (and primary selection) can be pasted into XWayland apps running inside gamescope. Text only, both CLIPBOARD and PRIMARY.

When gamescope runs nested under a Wayland compositor (KWin/Plasma, etc.), the clipboard bridge is one-directional today:

• Guest --> host works. As was implemented by PRs Add clipboard support from SDL backend #1449 and WaylandBackend: clipboard and primary selection support #1685.
• Host --> guest is implemented in this PR so it bridges the gap left by the aforementioned PRs.

Very quickly described, I decided to pick up the best applicable MIME type (to also filter out non-text clipboard content), forward that over a pipe until it reads EOF. Maximum clipboard size is set to 4MiB. It's then handed over to gamescope_set_selection and then handle_selection_request hands it to the guest.

One caveat by design is that the gamescope window will only accept input while the gamescope surface holds the host keyboard focus; meaning that it will only serve the intended loop of: focussing another application on the host machine, selecting text, copying the text, refocussing on the gamescope window, and then pasting it in. So the text selection events only fire when you refocus to gamescope, essentially. What this means is that if you are able to copy content while not losing the gamescope window's focus, it will not be pastable into gamescope.

[KogasaPls]

Relevant prior work / open PR: #2062

[WhyNotHugo]

Fixes: #1935