No more return false from onBeforeClose

You can no longer return false from onBeforeClose to prevent a view
from closing. This behavior was inconsistent with the rest of the
library, and would require far too much logic to respect in all cases.
Instead of returning false, you should handle your logic *before*
calling close on the view.

Fixes #1017.

:thought_balloon: Thoughts and considerations: Nothing happens between `before:close` and `close` now.