This avoids repeated dynamic memory allocations in dbusMessageHandler
methods and elsewhere due to repeatedly constructing temporary strings.
std::string_view is a C++17 feature but also part of the Library
Fundamentals TS and available in all our compilers.
QtDBus was the main component of xswiftbus' Qt dependency. This is the
first preparation step to get xswiftbus Qt free.
The new implementation is based on the low level libdbus library, which
was also in use by QtDBus itself. But instead of QtDBus, we use now a thin
C++ wrapper. To keep DBus handling async, libevent is used to monitor
timeouts and fds.
