Add confirm() to "Mark all as read?" button

[guidopetri]

Originally by @naps62 in #481:

I accidentally clicked "Mark as all read" instead of the "Refresh" button, causing all my unread entries to disapear.

This looks like a UI problem to me. The button is not really clear on it's purpose since there's only an icon (and a title attribute, but that only shows up if you hover it for a second).

Since it's such a no-turning-back action, I would say there should be some confirmation here