Handle shift-modifier if tab is pressed on System.Console and Windows drivers.

- This fixes backtab not working on Windows and System.Console.

Terminal.Gui/Drivers/NetDriver.cs

@@ -244,7 +244,7 @@ namespace Terminal.Gui {
 			case ConsoleKey.Escape:
 				return Key.Esc;
 			case ConsoleKey.Tab:
-				return Key.Tab;
+				return keyInfo.Modifiers == ConsoleModifiers.Shift ? Key.BackTab : Key.Tab;
 			case ConsoleKey.Home:
 				return Key.Home;
 			case ConsoleKey.End:

Terminal.Gui/Drivers/WindowsDriver.cs

@@ -604,7 +604,7 @@ namespace Terminal.Gui {
 			case ConsoleKey.Escape:
 				return Key.Esc;
 			case ConsoleKey.Tab:
-				return Key.Tab;
+				return keyInfo.Modifiers == ConsoleModifiers.Shift ? Key.BackTab : Key.Tab;
 			case ConsoleKey.Home:
 				return Key.Home;
 			case ConsoleKey.End: