Highlight and report non-existent include directories

[buhtz]

In this example the Include directory nextCloud does not exist anymore in the real file system.

This should be somehow mentioned to the user in several ways:

• Warning in the logs
• Warning in the systray/notify
• Highlight that directory in the GUI (e.g. color red) and add explanation as a tooltip

[buhtz]

Related to PR #1718 and Issue #1586 ?

[buhtz]

Close as (near) duplicate of #1586. See PR #1718 for further details about implementing a solution and obstacles discovered.