Move / rename local sync folder (e.g. `~/Nextcloud` -> `~/MyCloud`)

Initially I was sure there already has to be an issue for this, but I didn't find one, so here it comes.

### Expected behaviour
Being able to change NC base folder for account by UI

### Actual behaviour
You can't. Worse: If you rename it via finder/explorer/whatever, client explodes telling you it's unable to finde the NC folder for corresponding account, but doesn't offer relocate options or similar.

### Steps to reproduce
1. Develop the desire to rename base folder
2. Lie down, try not to cry - cry a lot.

### Workarounds
- Edit config file manually (potentially dangerous, uncomfortable)
- Delete / re-add account, triggering wizard (meh)

### Why should this be necessary
I admit this is rather rarely needed. But it is: Default name for NC client folder is "Nextcloud". The first time you may add a second account, you'll probably give it a meaningful name. This may result in something like "/home/User/Nextcloud" and "/home/User/NC-Work". Just reasonable to be triggered then, wanting to rename "Nextcloud" to "NC-Private".