Qt: Limit the amount of Follow Stream text.

QTextEdit + QTextDocument don't handle large amounts of data very well.
Truncate our displayed text after 2 MB.

Change-Id: I0b549a0b05a490272bc0d34af0f2c6661c548d0f
Reviewed-on: https://code.wireshark.org/review/5042
Reviewed-by: Gerald Combs <gerald@wireshark.org>