AdditionalTools validation & feedback

[andreww-msft]

Summary of the pull request

In Settings | Additional Tools, ensure the toolpath and name are valid, and provide user feedback when registering a tool (via a new timed message dialog in WindowHelper). Also only enable the Unregister button if the user selects at least one tool (and enable unregistering multiple tools at once).

Also, added tooltips to all the fields on the App Details page; suppress the external tools menuflyout if there are no external tools; and fixed the pin/unpin menu icon.

References and relevant issues

Fixed the following:

• Closes Settings | additional tools, the delete tool button is shown/enabled even if there are no tools #3040
• Closes Right click on the external Tools button shows an empty context menu (with no registered external tools) #3041
• Closes Settings | additional tools, insufficient visual feedback when adding a tool #3042
• Closes The "Pin" button for an external tool in the context menu has the wrong icon (shows UnPin).  #3043
• Closes The "app framework types" section needs a tooltip or label to explain it #3045
• Closes Settings | additional tools, insufficient input validation #3047

[krschau]

I edited your description with the magic syntax so GitHub would link everything together. Now when this closes, the issues it fixes will close.

[jaholme]

[zadesai]