Clean up UX

Hi! Excellent, simple plugin. Love it.

To make things clearer for users, I made some changes in my version. Hopefully they'll be helpful. This PR:

1. Checks for inactive widgets on page load and conditionally displays the button
1. Creates a confirmation dialog as an extra precaution (since there's no undo)
1. Leaves the Inactive Widgets section in place, even after deletion
1. Generally cleans up the copy to align with WordPress

Let me know if I can do anything else to help. Thanks!