file copy&paste using webdav

This re-uses the existing phodav server to share copied files with the guest.

See MRs for the other components for more details:

• spice-protocol!21 (closed)
• spice/linux/vd_agent!13 (closed)
• spice!93 (closed)
• https://gitlab.gnome.org/GNOME/phodav/-/merge_requests/16
• spice/win32/vd_agent!29 (closed)

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v2

• use existing channel instead of creating a new one
• use \0 as delimiter for VD_AGENT_CLIPBOARD_FILE_LIST instead of \n

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v3

• fix build without phodav
• support cutting files for nautilus and dolphin

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v4

• keep the original phodav dir structure (updated phodav MR)

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v5

• minor fix concerning retrying if the retrieved data is not in nautilus format

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v6

• extended commit messages

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v7

• added support for "x-special/gnome-copied-files" and "x-special/mate-copied-files", which means that cut/move should be working with more file managers