Cocoa: Add autorelease pools.

Cocoa: Add autorelease pools.

A couple of cases where we call Cococa APIs without
having an autorelease pool in place surfaced after
removing the global autorelease pool in 1a218a7.
(This happens when when Qt API is called before
app.exec() has started the Cocoa event loop.)

Add local autorelease pools to prevent memory leaks.

Change-Id: I0c4be3ff102aaff4539235857f95ab29fdbc9d70
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@nokia.com>