Fix DataPanel too small making data unreadable

[PhilippeM]

First, thanks for this very helpful plugin.
Using it, I noticed that when you have data in WebSocket request-response Sampler request data field that is long enough, it becomes unreadable in GUI.

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[PhilippeM]

https://bitbucket.org/pjtr/jmeter-websocket-samplers/commits/47b4078f44ed0e110234345726388f279e9b050f