Get the system bus higher up in the application, so the same system bus
connection can be shared between different parts of the application if
needed in future.
This also means the synchronous I/O needed to connect to the bus is done
before the application UI is shown, which prevents it unnecessarily
blocking initialisation of the `MctUserControls` widget.
Signed-off-by: Philip Withnall <withnall@endlessm.com>
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