[IDE] allow delete missing files from project

- After a file is deleted from Finder outside VS Mac, right click on the file will show "Remove" instead of "Delete" and a dialog will confirm user's choice to remove the file and its child from the project. If any child exists, it will be removed (not deleted)

(Should we directly delete the child?)

Fixes: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1051207

Author: ketttY

main/external/debugger-libs

@@ -285,35 +285,34 @@ public override void DeleteMultipleItems ()
 			}
 
 			string question;
-			bool allLinkedFiles = CheckAllLinkedFile (files);
+			bool fileExists = CheckAnyFileExists(files);
 
-			if (allLinkedFiles) {
+			if (CheckAllLinkedFile (files)) {
 				RemoveFilesFromProject (false, files);
 			} else {
-
-				bool filesExist = CheckAnyFileExists (files);
-
+				string behavior = fileExists ? "delete" : "remove";
 				if (hasChildren) {
 					if (files.Count == 1)
-						question = GettextCatalog.GetString ("Are you sure you want to delete the file {0} and " +
-															 "its code-behind children from project {1}?",
-															 Path.GetFileName (files [0].Name), files [0].Project.Name);
+						question = GettextCatalog.GetString ("Are you sure you want to {0} the file {1} and " +
+															 "its code-behind children from project {2}?",
+															 behavior, Path.GetFileName (files [0].Name), files [0].Project.Name);
 					else
-						question = GettextCatalog.GetString ("Are you sure you want to delete the selected files and " +
-															 "their code-behind children from the project?");
+						question = GettextCatalog.GetString ("Are you sure you want to {0} the selected files and " +
+															 "their code-behind children from the project?", behavior);
 				} else {
 					if (files.Count == 1)
-						question = GettextCatalog.GetString ("Are you sure you want to delete file {0} from project {1}?",
-															 Path.GetFileName (files [0].Name), files [0].Project.Name);
+						question = GettextCatalog.GetString ("Are you sure you want to {0} file {1} from project {2}?",
+															 behavior, Path.GetFileName (files [0].Name), files [0].Project.Name);
 					else
-						question = GettextCatalog.GetString ("Are you sure you want to delete the selected files from the project?");
+						question = GettextCatalog.GetString ("Are you sure you want to {0} the selected files from the project?", behavior);
 				}
-		 
-				var result = MessageService.AskQuestion (question, GetDeleteConfirmationButtons (filesExist));
-				if (result != AlertButton.Delete)
-					return;
 
-				RemoveFilesFromProject (true, files);
+				var result = MessageService.AskQuestion (question, new [] { AlertButton.Cancel, fileExists ? AlertButton.Delete : AlertButton.Remove });
+
+				if(result == AlertButton.Cancel)
+					return;
+				else
+					RemoveFilesFromProject (fileExists, files);
 			}
 
 			IdeApp.ProjectOperations.SaveAsync (projects);
@@ -323,18 +322,13 @@ public override void DeleteMultipleItems ()
 		[AllowMultiSelection]
 		void OnUpdateDeleteMultipleItems (CommandInfo info)
 		{
+			var files = new List<ProjectFile> ();
 			foreach (var node in CurrentNodes) {
 				var pf = (ProjectFile)node.DataItem;
-				if (pf.IsLink)
-					info.Text = GettextCatalog.GetString ("Remove");
+				files.Add (pf);
 			}
-		}
-
-		static AlertButton [] GetDeleteConfirmationButtons (bool includeDelete)
-		{
-			if (includeDelete)
-				return new [] { AlertButton.Cancel, AlertButton.Delete };
-			return new [] { AlertButton.Cancel };
+			if( !CheckAnyFileExists(files) )
+				info.Text = GettextCatalog.GetString ("Remove");
 		}
 
 		[CommandHandler (ProjectCommands.ExcludeFromProject)]
@@ -379,7 +373,7 @@ public void RemoveFilesFromProject (bool delete, List<ProjectFile> files)
 
 				// Delete file before removing them from the project to avoid Remove items being added
 				// if the project is currently being saved in memory or to disk.
-				if (delete && !file.IsLink)
+				if (delete && !file.IsLink && File.Exists (file.Name))
 					FileService.DeleteFile (file.Name);
 				project.Files.Remove (file);
 			}