UI validation enhancements

[sfmskywalker]

This PR adds workflow publishing validation enhancements.

=== auto-pr-body ===

Summary:
This pull request includes the following enhancements:

• Added IMediator interface to facilitate publishing notifications and subscription to notifications
• Added tabler icons and heroicons to provide icons for Elsa studio
• Added summary comments to describe interface, Elsa Studio icons and heroicons classes
• Added result class to hold successful or failure values and used to check if the result was successful or not
• Added methods such as Subscribe and Unsubscribe for particular notifications, Unsubscribe for any notification, NotifyAsync, and methods of type Func<TSuccess, Task>/Action<TSuccess> and Func<TFailure, Task>/Action<TFailure>.
• Added [CascadingParameter] attribute to the DragDropManager property, [Parameter] attributes to the workflow definition and workflow definition updated properties, and constructor and properties to ActivityDisplaySettings class.
• Updated return type of CreateNewDefinitionAsync from Task<WorkflowDefinition> to Task<Result<WorkflowDefinition, ValidationErrors>> and added two new properties to DefaultActivityColors class.
• Removed ActivityIcons class.
• Updated OnFileSelected to set the _isDirty flag to false, and OnCreateWorkflowClicked to use the Result object to handle the result from the CreateNewDefinitionAsync operation.

List of Changes:

• Added IMediator interface to facilitate publishing notifications and subscription to notifications
• Added tabler icons and heroicons to provide icons for Elsa Studio
• Added summary comments to describe interface, Elsa Studio icons and heroicons classes
• Added result class to hold successful or failure values and used to check if the result was successful or not
• Added methods such as Subscribe and Unsubscribe for particular notifications, Unsubscribe for any notification, NotifyAsync, and methods of type Func<TSuccess, Task>/Action<TSuccess> and Func<TFailure, Task>/Action<TFailure>.
• Added [CascadingParameter] attribute to the DragDropManager property, [Parameter] attributes to the workflow definition and workflow definition updated properties, and constructor and properties to ActivityDisplaySettings class.
• Updated return type of CreateNewDefinitionAsync from Task<WorkflowDefinition> to Task<Result<WorkflowDefinition, ValidationErrors>> and added two new properties to DefaultActivityColors class.
• Removed ActivityIcons class.
• Updated OnFileSelected to set the _isDirty flag to false, and OnCreateWorkflowClicked to use the Result object to handle the result from the CreateNewDefinitionAsync operation.

Refactoring Target:

• Extract methods of type Func<TSuccess, Task> and Action<TSuccess> and Func<TFailure, Task> and Action<TFailure> into their own functions for improved readability.
• Rename the methods of type Func<TSuccess, Task> and Action<TSuccess> and Func<TFailure, Task> and Action<TFailure> to be more expressive and descriptive of the functionality they represent.
• Create a generic SaveChangesAsync method for both the SaveAsync and PublishAsync methods, that receives a bool readDiagram, bool showLoader and bool publish parameter, and returns a Result<WorkflowDefinition, ValidationErrors> type.
• Create a generic method to handle errors, since the same logic is repeated when calling SaveAsync, PublishAsync and RetractAsync methods.
• Rename the NotifyWorkflowChangedAsync method to indicate the task it performs (e.g. 'InitializeWorkflowAsync`) and include all the logic it previously contained (selecting the activity, etc.).
• Make use of async/await in OnCreateWorkflowClicked to ensure that the result of the CreateNewDefinitionAsync operation is handled properly.
• Consider using a try/catch block for the RetractAsync operation in OnRetractClicked to handle any potential errors.