A copy of the clipboard was always being, changes would re-read it.
Now read the clipboard on request. This avoids having to keep a copy
of the clipboard in memory as well as the need to keep a thread
to running to read the clipboard for each data-offer.
To prevent a deadlock when pasting from Blender's own clipboard.
- Sending the clipboard (using write(..)) runs in a background thread.
- Reading the clipboard uses a thread that performs round-trips to the
Wayland server to prevent until the read is complete.
This is an update to [0] that resolves the deadlock.
[0]: c03838dbc8