Allow setting a custom default location in nextcloud.cfg

It would be useful to allow specifying a custom location of the [defaultClientFolder](https://github.com/nextcloud/desktop/blob/master/src/libsync/theme.cpp#L554) in the nextcloud.cfg config file. Since it also allows absolute paths to be specified. This can be useful in environments where the admin wants to make sure that user created connections are not showing up in the user home, because it is located on a network share.