Improve user data support in ObjectData

[danieldg]

This allows using Proxy::data on custom ObjectData implementations, and simplifies the implementation of QueueProxyData.

[codecov[bot]]

Codecov Report

Merging #535 (e93fad7) into master (2dd5e5d) will decrease coverage by 0.11%. The diff coverage is 84.00%.

@@            Coverage Diff             @@
##           master     #535      +/-   ##
==========================================
- Coverage   76.90%   76.78%   -0.12%     
==========================================
  Files          50       50              
  Lines        6906     6892      -14     
==========================================
- Hits         5311     5292      -19     
- Misses       1595     1600       +5     

Flag	Coverage Δ
main	62.34% <4.00%> (+0.12%)	:arrow_up:
test--	81.99% <84.00%> (-0.17%)	:arrow_down:
test--server_system	63.68% <84.00%> (-0.54%)	:arrow_down:
test-client_system-	71.55% <84.00%> (+0.18%)	:arrow_up:
test-client_system-server_system	52.31% <84.00%> (+0.17%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
wayland-backend/src/client_api.rs	90.47% <0.00%> (-4.53%)	:arrow_down:
wayland-client/src/event_queue.rs	62.09% <95.23%> (-2.03%)	:arrow_down:
wayland-scanner/src/client_gen.rs	99.19% <100.00%> (ø)
wayland-backend/src/rs/client_impl/mod.rs	80.11% <0.00%> (-0.76%)	:arrow_down:
wayland-backend/src/rs/wire.rs	87.15% <0.00%> (-0.56%)	:arrow_down:
wayland-backend/src/sys/server_impl/mod.rs	75.78% <0.00%> (-0.54%)	:arrow_down:
wayland-backend/src/rs/server_impl/client.rs	76.43% <0.00%> (-0.19%)	:arrow_down:
wayland-backend/src/rs/server_impl/handle.rs	71.18% <0.00%> (+0.69%)	:arrow_up:
wayland-backend/src/sys/client_impl/mod.rs	74.75% <0.00%> (+0.70%)	:arrow_up:

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