Refactoring: replaced proprietary timed dialog in ConfirmVirtProposal

using Popup.TimedAnyQuestion. It simplifies and DRY the code. Moreover
user now is informed that dialog has timeout (it wasn't clear before)
Merge pull request #179 from mchf/refactoring_ConfirmVirtProposal

Refactoring: replaced proprietary timed dialog in ConfirmVirtProposal