Qt: Add Decode as in Show Packet Bytes

Qt: Add Decode as in Show Packet Bytes

Add an option to decode the packet bytes from base64 or zlib compressed.
Also add configurable start byte and end byte to make it possible to
decode a subset of bytes.  It's also possible to select a range in ASCII
view and select "Show selected" from the context menu to make a subset.

In ASCII view a null terminator is replaced by UTF8 symbol for NULL,
and a CR is replaced by UTF8 symbol for carriage return.  This is done
to make it possible to "Show selected" from the context menu.

Change-Id: Ie03c9912c304c121af6ca9e998a6e8445b5382c5
Reviewed-on: https://code.wireshark.org/review/13958
Petri-Dish: Stig Bjørlykke <stig@bjorlykke.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>