Implement integration tests for desktop app

[pverscha]

We should implement integration tests for the desktop app using the "Spectron" framework. I propose to choose Jest as the test environment to control the output of Spectron, since that is the same testing environment we are using in Unipept web components.

The integration tests should be executed automatically by a GitHub action and merging code with master should only be allowed if the integration tests succeed, since a merge to master automatically builds a new version of the application.