Add support for passing page instances as "page" options to window switching related methods

[geb-bot]

Originally created by Marcin Erdmann.

• Browser.withWindow()
• Browser.withNewWindow()

[geb-bot]

Originally posted by Varun Menon.

This issue has been fixed as part of pull request:
apache/groovy-geb#86

[geb-bot]

Originally posted by Marcin Erdmann.

Thanks for the reminder, I forgot to close it.

[geb-bot]

Resolved