json: Use GString instead of QString to accumulate strings

json: Use GString instead of QString to accumulate strings

QString supports modifying its string, but it's quite limited: you can
only append.  The remaining callers use it for building an initial
string, never for modifying it later.

Change parse_string() to do build the initial string with GString.
This is another step towards making QString immutable.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201211171152.146877-18-armbru@redhat.com>