Qt: Refactor ByteViewText.

Instead of subclassing QTextEdit and filling it with the entire contents
of our tvbuff, subclass QAbstractScrollArea and draw text by hand only
when needed. The new code should be *much* faster.

Some code based on QHexView by Even Teran
(https://code.google.com/p/qhexview/).

To do:
- Finish the bit view implementation.

Change-Id: Ie44de6870d80711cd44324521a17ab76bcefe5e5
Reviewed-on: https://code.wireshark.org/review/4922
Reviewed-by: Gerald Combs <gerald@wireshark.org>