Adding the folder name in the tooltip of the visualisation

[loub]

I wanted to be able to view for a message in a thread in which folder it is. So I added it to the tooltip. I'm pretty new to both Thunderbird plugins and github, so excuse me if I did things wrong.

A few comments about it:

• I wanted to see the whole path. And the function getFolder already provides that, but it also includes the account in the string, and makes it very long. So I created a dedicated function called getFolderName which returns only the current folder name.
• in the getFolderName function, I'm not sure when it can happen that the message is null. So in such cases, I don't know what it should return.
• I don't speak German, so I put the same string in German