Qt: Fix -Wshorten-64-to-32 warnings

Qt: Fix -Wshorten-64-to-32 warnings

Fix some warnings when building with -Wshorten-64-to-32 flag for
C++ code.

Fixes for warnings from QList, QTimer and QVector has been pushed
upstream, so some time we may be able to enable this flag for C++.

Change-Id: Iae7457f9afc469c63f3edbe23dbf272b5c6c9e5e
Reviewed-on: https://code.wireshark.org/review/20310
Petri-Dish: Stig Bjørlykke <stig@bjorlykke.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>