Support configurable timeout with alert related keywords #942

In S2L 1.8 alert related keywords use hard coded one second wait when looking for alerts. Currently in SL 3.0 beta 3 the timeout has been increased to 5 seconds but is still hard coded. As #917 already proposed, this timeout should be configurable.

We've made a decision that these keywords will use the same timeouts as Wait ... keywords by default, but they also have timeout=None argument to override.

We also have #933 about cleaning up alerts in general, and that involves deprecating several keywords. The support for custom timeouts should be only added to keywords that are going to be preserved.