Add a search function to the folders column on the left

Actually we have exactly this already for labels. We have to think if we extend this to folders.

The thing is we also have some plans to make the main search smarter. So for example if you search there for something that looks like a folder you have it should be possible to quickly switch to this folder.

This has been suggested before here: Searching by label and we have concrete plans to implement that. As usual I can’t give an ETA though.