Moves all the stuff that is needed to start DBus on macOS (which for some reason it needs several steps, involving the use of `launchctl`) into a single `startDBusDaemon()` function. Before, it was spread into `kdeconnectconfig.cpp`, `indicatorhelper_mac.cpp` and `dbushelper.cpp`.
It also removes checking for an existing DBus daemon and always starts our own, since in most cases we couldn't connect to it anyway. This, together with removing the sleep in the retries when polling for the DBus daemon from 3s to 100ms, makes the startup much faster, so I removed the loading splash screen.
By commenting out the parameter name, we get compile-time checks
Also, we can omit them for slots and Qt will not forward the parameters.
In case we had TODOs next to the code, I kept the Q_UNUSED statements
for now.
