Better patch to replace !218.

- Auto and quick detection of previous D-Bus instance;
- Remove private D-Bus compile definition, only use it on macOS without an existing D-Bus instance;
- Safe reboot after crashes because the indicator is not relating on the kdeconnectd to run a D-Bus session;
- Safe exit after clicking on `Quit` in the systray.


More details in commit logs:

Only enable private D-Bus on macOS because the other platforms do not
need them.
The app should be able to easily detect the session bus from the env
DBUS_LAUNCHD_SESSION_BUS_SOCKET from launchd through launchctl.
Because https://gitlab.freedesktop.org/dbus/dbus/-/blob/master/dbus/dbus-sysdeps-unix.c#L4392
shows that it is the only probing method on macOS with launchd.

The D-Bus session bus can be easily found from launchd/launchctl
with DBUS_LAUNCHD_SESSION_BUS_SOCKET env. It can be an external one
(installed from HomeBrew) or an internal one (launched by a previous
instance followed by a cras...