Possibility to use the client in test-mode

[dotarmin]

Under settings there should be an option for running the client in a test-mode that enables features such as issue #240 authored by @didikunz. This feature set could be expanded with more nice-to-have features when test-mode is enabled.

When test-mode is enabled under settings (or selected), the end user should see it visually in some way, preferable maybe a bar at the top of the UI coloured in orange or so.

[didikunz]

Nice idea, I will think about other "nice to have" features, that cound run under this umbrella.