Fix the keyboard key held state occurring due to bad network conditions

[PMohanJ]

Reference the issue numbers and reviewers
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@ehfd
Fixes #145

Explain relevant issues and how this pull request solves them
This pull request resolves the issue by replicating the solution provided in the reference https://github.com/Xpra-org/xpra/blob/a5a13a993139d9d48a60734e7bab3ec9d53c919a/xpra/server/mixins/input.py#L257.

Describe the changes in code and its dependencies and justify that they work as intended after testing
The changes are around the web interface and python server, where the client sends key-repeat event for the keys that are in key pressed state to the server. The server monitors these key-repeat event for the held keys and manages them based on the time interval of the key-repeat event sent by the client. When there's a unstable network, the server would just release those keys.

Describe alternatives you've considered
NA

Additional context
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• I confirm that this pull request is relevant to the scope of this project. If you know that upstream projects are the cause of this problem, please file the pull request there.
• I confirm that this pull request has been tested thoroughly and to the best of my knowledge that additional unintended problems do not arise.
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[ehfd]

All good, except that the internal could be fine-tuned based on our experiences. Balance between excess type strokes under unstable networks vs responsiveness in interactive control (such as gaming).

@PMohanJ