Make the desktop client not use dbus on Windows and MacOS

Currently KDE Connect needs DBus so that kdeconnect-indicator, kdeconnect-app and other binaries can talk to the kdeconnectd binary.

DBus is not a thing outside of Linux/BSD, so we have to start our own dbus process. This has some issues, at least:

• We run several different processes (including dbus) that we have to keep in sync (eg: when the user quits kdeconnect we have to stop all of them, if one crashes and is restarted it needs to not try to start a second instance of the others, etc.)
• If some other app on the system also uses dbus and brings a different version or a different configuration, one of the two apps might fail to start (the one started in second place).

Furthermore, we don't really need this since we can run everything in the same process (unlike in Linux where we have the plasmoid, file browser extensions, etc. that are external).

This can be a bit tricky to change though.