http2: select fields from Decompressed header tab when possible

http2: select fields from Decompressed header tab when possible

The "Decompressed header" tab contains some human-readable text, but no
field was associated with it. Instead, the fields were attached to raw
compressed headers which, all with the same offset and length.

Ensure that each byte in the decompressed header tab is accounted for.
The only fields that are still pointing to the raw compressed buffer is
the http2.header field (covering a full raw header), the representation
type (a few bits, at most 1 octet) and the index length (guessed length,
an exact value is probably not worth the cpu cycles).

Change-Id: Ic0118e9ed583841a2d353f8b8c28dcafea3401f2
Reviewed-on: https://code.wireshark.org/review/24660
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>