DismissOnClickOutside property added to DropDownButton

[MuhammadSulaiman001]

Hi, I have been using DropDownButtons and SplitButtons to show dialogs by setting the content of these buttons equals to the dialog's content, this results in a pretty slide-down show behavior.. This is a sample snippet

<fluent:DropDownButton
      Header="Header"
      IsDropDownOpen="{Binding IsDialogOpen, Mode=TwoWay}"
      SizeDefinition="Large, Large, Large">
      <fluent:DropDownButton.LargeIcon/>
      <views:MyDialog/>
</fluent:DropDownButton>

with this, I only miss one thing, prevent closing the drop-down-dialog when clicking outside, so, I have added a property named DismissOnClickOutside to DropDownButton class.. setting it to false will prevent closing the dialog unless the user clicks over the DropDownButton

Notes:

1. This property does not affect other properties like IsDropDownOpen or ClosePopupOnMouseDown
2. I could not have just Inherit DropDownButton and make my own DropDownButton to achive my purpose because I needed to add a check code inside PopupService.cs
3. The default value of DismissOnClickOutside is true, which leads to the normal behavior (i.e. closing the popup when clicking outside)

[batzen]

Thanks for contributing!