Qt: ByteViewText hover information.

When the user hovers over a byte view field, highlight it and show a
description in the status bar.

Add a "byte" status bar context and fix a label stack pop bug.

Keep proto_find_field_from_offset from matching generated items.
Otherwise hovering and selecting finds things like GeoIP entries and
checksum validation information. This affects the GTK+ UI as well.

Change-Id: Ic81c0d8159510a72d30c41f961807d8a48d05e16
Reviewed-on: https://code.wireshark.org/review/4943
Reviewed-by: Gerald Combs <gerald@wireshark.org>