qt: use qstring_strdup

Replaces the more verbose `g_strdup(foo.toUtf8().constData())` by
`qstring_strdup(foo)`. While at it, plug a minor memleak in
MainWindow dialogs.

Change-Id: I32b53f972b4e3998ac9beabd98647a381b7b42e3
Reviewed-on: https://code.wireshark.org/review/5531
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>