Rework how notifications are shown

Rework how notifications are shown

Users found `new Notification(message)` too intrusive. This commit shows
notifications in a `<statuspanel>` instead, like the "URL popup" (shown when
hovering or focusing links), but on the opposite side.

When pressing `n` or `N` and the search matches a link, the URL popup is shown.
Putting the VimFx notification on the opposite side allows both to be visible at
the same time.

Notifications are shown until you click them, press a key or switch tab.

See #576.