Warn user when content will get public on plugin deactivation

[druxstr]

What problem does this address?

It's really easy to set custom visibility for some blocks and to forget about it.

If you do so, you may deactivate the plugin and make public some content without noticing.

What is your proposed solution?

If at least one block has a visibility option set, warn user before plugin deactivation. The message could be like :

You have at least one block with a visibility option set. Deactivating this plugin will make this content public.

It doesn't have to specify which blocks or where to find the blocks (even if this would be nice).