Verification trigger attributes to inherit from WaitingTriggerAttribute

[YevgeniyShunevych]

The following trigger attributes should inherit from WaitingTriggerAttribute instead of base TriggerAttribute:

• VerifyTitleAttribute
• VerifyH1Attribute - VerifyH6Attribute
• VerifyExistsAttribute
• VerifyMissingAttribute
• VerifyContentAttribute
• VerifyContentMatchesAttribute

Now the above attributes will get Timeout and RetryInterval properties from WaitingTriggerAttribute that they should use for verification. So the user will be able to set one or both of these properties to, for example, increase the verification time for slow pages.