Enable adding external tools to the External Tools menu

[andreww-msft]

Summary of the pull request

Previously, we were just adding each external tool to the toolbar. Per spec, we're now adding them to the ExternalTools menu as well.

As part of this, there was some refactoring of the ExternalTool and WindowHelper classes to get the different image formats required for both the toolbar and the menu without duplicating code. Also, as you can't databind to MenuFlyoutItem, and the ExternalTool icon image is generated asynchronously, we're handling the PropertyChanged event in code, so we can update the icon when it gets set.

Also implemented unregistering an ExternalTool from either the bar button or the menuitem for the tool.

Implemented the ExternalTools button Click handler to open the Settings UI and navigate to the Additional Tools section. As part of this, added 2 derived NavLink types, as their usage in Settings and in the ExpandedView are sufficiently different, and updated the SettingsToolWindow to match. Because we can open Settings without the ExpandedView, I've reinstated the original behavior of closing all open child windows when the Bar window closes (right now, this is just the settings window).

Unrelated fixes:

• Reinstated the ModulesPage fields for BaseAddress and EntrypointAddress, as these no longer fail.
• Removed redundant INotifyPropertyChanged from TargetAppData, as it derives from ObservableObject.