Wayland backend: add host-to-client clipboard sync

[nseguin42]

This PR adds the other half (host-to-client direction) of clipboard support for the Wayland backend (the client-to-host direction being added in e17aec4). In other words, it allows the gamescope client to paste from the host's clipboard.

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The implementation is based on Emersion's blogpost from 2020: "Wayland clipboard and drag & drop" . In the post, Emersion writes (emphasis mine):

Note that we’re performing blocking write calls in data_source_handle_send. This could potentially stall the Wayland event loop if we had more data to write (overflowing the kernel buffer). A real client would perform non-blocking writes instead.

I'm not sure how to go about making this change, so I'd appreciate some input here.

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Only the clipboard is supported (not the primary selection). Since the client-to-host direction supports the primary selection (c.f. ef1e8db), we should probably add this too.

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Only the "text/plain" MIME type is requested. Should we request different MIME types?

[MithicSpirit]

Thanks for doing this, I've been meaning to do it eventually, but never got around to it.

To prevent stalling too much on reads, I just prevented it from reading more than once, and print out a message when it doesn't fit (see line 2516).

I think it would be good for this to support all the same MIME types that the source supports (see lines 1228–1232 and 1240–1244; we should probably refactor these together so we don't triplicate the list of MIME types). However, more importantly, I think it should check the offered MIME types before getting the data. Currently, if I copy an image to the clipboard and try to paste it into gamescope, it just prints out some garbage, while I think it should either empty the internal clipboard or leave it unmodified.