Implement timeout for fetchNextReponse

[julianweise]

Currently, the framework does not time out if fetchNextResponse does not deliver a response.

[felixrindt]

Can reproduce. When clicking "Update" without internet connection, the app tries to fetch emails and wont stop alone after waiting for at least 20 seconds.

When I aborted using Alt+., the connection was broken: When the internet connection is enabled again, I get a NumberParserError, which is related to #29.

[JuliaJoch]

Possible Solution: When update is called we need to check the connection before fetching the Folders and Mails. There already is a Method that tests the connection on the Endpoint.