[Cocoa] Properly port WKUIDelegate methods
[Cocoa] Properly port WKUIDelegate methods

Manually assign the missing block signatures of the completionHandler
arguments (idea credits: https://github.com/munki/munki/commit/b9944e9).

r0x0r/pywebview,r0x0r/pywebview,r0x0r/pywebview,r0x0r/pywebview,r0x0r/pywebview