Fix: Fixed issue where opening and closing modals quickly may lead to a crash

[ferrariofilippo]

Resolved / Related Issues

• Were these changes approved in an issue or discussion with the project maintainers? In order to prevent extra work, feature requests and changes to the codebase must be approved before the pull request will be reviewed. This prevents extra work for the contributors and maintainers.
  Closes Bug: Opening and closing modals quickly may lead to a crash #11804

Details

• I couldn't use the solution proposed in the related issue (creating a wrapper) since some dialogs may be at random times (paste) and that would lock most of the actions
• Now in UIHelpers there's a bool that is set to true when the app is ready to show a new dialog
• BaseUIAction is used to reduce code duplication (UIHelpers.PropertyChanged)
• I removed DialogExtension.cs since it was used only once and there was a method that did the same thing
• I removed all but one SetContentDialogRoot() since they share the same code. (Note this method is invoked by TryShowAsync() so everything works same as before)

Others

• Do we really need DialogResult? It has the same values as the predefined enum ContentDialogResult

Validation
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• Did you remove any strings from the en-us resource file?
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  • Was this change approved?
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  1. Open app and select a file
  2. quickly click Del and Esc