Use native WebDriver way of handling alerts

[geb-bot]

Originally created by Marcin Erdmann.

This would greatly simplify the code in AlertAndConfirmSupport as well as would allow to avoid issues in situations where page is reloaded after an alert/confirm has been dismissed and where you cannot access the alert/confirm message.

[erdi]

This has been reverted and reopened because some drivers seem to not implement the native way of handling alerts correctly or their implementation is not fully stable.

[erdi]

Given the incompatibilities between browsers in native alerts handling I think it's better to keep the compatibility layer Geb provides for the price of not being able to access the alert/confirm message when page is reloaded after an alert/confirm has been dismissed.