Let each interaction provide its own dispatcher page title

The interaction dispatcher gives every parked interaction step the same hard-coded page title, "Continue", from InteractionController::title(). A step cannot name its own page, so a payment step, a form step and a message step all show the generic "Continue" heading.

Proposed resolution

Add an optional title() method to InteractionInterface, defaulting to NULL in InteractionBase. Change InteractionController::title() to resolve the parked interaction the same way step() does and return its title(), falling back to "Continue" when the interaction has none or the request carries no valid token. Each interaction can then title its own page (for example a payment step "Complete your payment"); interactions that do not override it keep the generic default.

Remaining tasks

Review the merge request.