Refactor macOS startup code

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.