feat(viewcontroller): adds onWillDismiss callback (0 delay)

[manucorporat]

Short description of what this resolves:

#6702

Changes proposed in this pull request:

• Adds a new onWillDismiss callback that is called before the animation begins. (it has the same signature than onDismiss)

Ionic Version: 2.x

Fixes: #6702

[adamdbradley]

While we're at it, we should rename onDismiss to onDidDismiss to have the same pattern as our lifecycle events. And if we are going to do that, we can deprecate onDismiss and have it still work, but print a console.warn() to use onDidDismiss.

[manucorporat]

@adamdbradley I totally agree with you. Tomorrow I update the PR!