Currently, the failure to connect to an MTP device results in the UI displaying
an open device that appears empty. This change introduces a method
ConnectedDevice::ConnectAsync() that is expected to handle any connecting tasks
that could block asynchronously. Upon completion, this emits a ConnectFinished
signal that indicates success or failure. The row in the UI is only updated
after the successful response is received. Upon failure, DeviceManager will
clean up and the row in UI is left in the pre-connect state.
Currently, only the MtpDevice utilizes this mechanism. All other devices use a
default implementation that immediately reports success.
GPodLoader and MtpLoader Error signals were connected to Error signals in their repective device classes, but the actual signal definition in ConnectedDevice was removed in a refactor several years ago. This change adds LoaderError slots to these device classes and reports the error in the manner of the refactored code.