Make Nextcloud optional

[dshoreman]

Not everybody uses Nextcloud. If the respective config options aren't specified, we should default to -c, --clipboard mode so that getting started is easier.

This will likely require a slight reworking of the Yad config UI, or at the very least some added text stating that it's safe to leave certain things blank.

In an ideal world, you should be prompted as to whether you have or want to use Nextcloud before even given the option to configure it. That will also affect more generic options though (such as rename, which is currently the only one).