Summary:
The use of Q_FOREACH is advised against (https://doc.qt.io/qt-5/qtglobal.html#Q_FOREACH) since Qt 5.7 and will eventually be removed from Qt.
I replaced all occurrences with the range-for loop introduced in C++11 (except for the one in daemon.cpp in deviceIdByName which might have a bug / typo in it).
I added const to the container or casted it with qAsConst when appropriate to avoid unnecessary copies.
(This is my first submission. I did all the unit tests, and they all passed but I don't know how to show it here.)
Reviewers: #kde_connect, nicolasfella, apol
Reviewed By: #kde_connect, nicolasfella, apol
Subscribers: albertvaka, apol, nicolasfella
Tags: #kde_connect
Differential Revision: https://phabricator.kde.org/D6724
