ContentDialog resize improvements

[IvanDmitriev1]

Improved resizing methods in the ContentDialog and MessageBox.
Added ContentDialog events.
Added ShowAlertAsync, ShowSimpleDialogAsync methods to IContentDialogService.

Pull request type

• Update
• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes

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[cla-bot]

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[cla-bot]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have the users @IvanDmitriev1 on file. In order for us to review and merge your code, please contact the project maintainers to get yourself added.