Dropdown Added

  -AdvancedEntry Completed event fixed,
  -AdvancedEntry TextChanged event added,
  -AdvancedEntry Annotations updated
  -AdvancedEntry BindableProperties added:
     *ValidationMessage,  
     *IgnoreValidationMessage
     *PlaceholderColor (Can be given PlaceholderColor = "{DynamicResource YourColor}"  )
  -Dropdown added, with properties:
     * Title
     * IconImage
     * FontFamily
     * BackgroundColor
     * BackgroundColor
     * Color
     * TextColor
     * AnnotationColor
     * TitleColor
     * BorderColor
     * CornerRadius
     * Placeholder
     * IsRequired
     * IsValidated
     * ValidationMessage

     <input:Dropdown Title="Chosse an option below:"
                            TitleColor="Black"
                            ValidationMessage="This field is required" 
                            AnnotationColor="Accent" 
                            IsRequired="True" 
                            BorderColor="Black" 
                            Color="BlueViolet"
                            Placeholder="Choose one" 
                            ItemsSource="{Binding MyList}" 
                            SelectedItem="{Binding SelectedItem}" />