window: don't dismiss password popover when loading page

Since !1337 (merged), when loading a new page in the web view, all permission request popovers are dismissed. This is correct for standard permission request popovers that are backed by WebKit permission requests, but it doesn't make sense for password popovers. It results in us dismissing the Save Password? request almost immediately after it is presented to the user. Instead, keep it around indefinitely.

Fixes #2113 (closed)