Cocoa: Establish pattern for accessing globals

Use a static QCocoaIntegration pointer instead of
QGuiApplication. This removes the need to call out
of the platform plugin as well as the casting from
"platform" to "cocoa" types.

Change-Id: If432b3567811223b73a67548e475e07d63635b73
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>