Use IWindowInfo.Handle to retrieve window handle

(PR re-issued after rebase fail at https://github.com/mono/MonoGame/pull/2166)

OpenTK 1.1 offers a public GameWindow.WindowInfo.Handle property. This patch removes reflection from the startup path in favor of accessing the Handle property directly.

This patch depends on https://github.com/kungfubanana/MonoGame-Dependencies/pull/43