Adds a popup ContextMenu feature. Implements ContextMenu for TextField. (#1615)

* Implementing ContextMenu feature to being popup by keyboard or mouse.

* Implements ContextMenu into TextField.

* Ensures the context menu's right and bottom frame from being greater than the container.

* Added Host property. Improving scenario and unit tests.

* Only draw the RightTee if it is at the end of the menu.

* Implements cursor visibility on TextField.

* Fixes the sub-menu not showing.

* Avoids draw the menu help and shortcut if there no available space.

* Remove reference for the MenuClosing event.

* UpdateCursor must only run after the ScreenBuffer is initialized to use the cursor visibility.

* Implements Resized event on Toplevel class.

* Prevents writing overlay on the menu.

* Covering more unit tests.

* Changing from Views to Core namespace.

* Implementing MenuClosingEventArgs and MenuAllClosed event.

* Only close the menu if it's open.

* Implementing localization for en-US and pt-PT to the FileDialog.

* Implementing localization for en-US and pt-PT on the TextField context menu.

* Fixes a bug where DeleteSelectedText is updating the Text before operation completion.

* Added a method to get all the supported cultures from the Terminal.Gui.

* Improving context menu and adding more unit tests.

UnitTests/TextFieldTests.cs

@@ -1092,15 +1092,20 @@ namespace Terminal.Gui.Views {
 			tf.CursorPosition = tf.Text.Length;
 			Assert.True (tf.ProcessKey (new KeyEvent (Key.Backspace | Key.CtrlMask, new KeyModifiers ())));
 			Assert.Equal ("to jump between text ", tf.Text);
+			Assert.True (tf.ProcessKey (new KeyEvent (Key.T | Key.CtrlMask, new KeyModifiers ())));
+			Assert.Equal ("to jump between text ", tf.SelectedText);
+			Assert.True (tf.ProcessKey (new KeyEvent (Key.D | Key.CtrlMask | Key.ShiftMask, new KeyModifiers ())));
+			Assert.Equal ("", tf.Text);
 		}
 
 		[Fact]
 		[AutoInitShutdown]
 		public void Adjust_First ()
 		{
-			TextField tf = new TextField ();
-			tf.Width = Dim.Fill ();
-			tf.Text = "This is a test.";
+			TextField tf = new TextField () {
+				Width = Dim.Fill (),
+				Text = "This is a test."
+			};
 			Application.Top.Add (tf);
 			Application.Begin (Application.Top);
 
@@ -1115,5 +1120,63 @@ namespace Terminal.Gui.Views {
 				return item;
 			}
 		}
+
+		[Fact, AutoInitShutdown]
+		public void DeleteSelectedText_InsertText_DeleteCharLeft_DeleteCharRight_Cut ()
+		{
+			var newText = "";
+			var oldText = "";
+			var tf = new TextField () { Width = 10, Text = "-1" };
+
+			tf.TextChanging += (e) => newText = e.NewText.ToString ();
+			tf.TextChanged += (e) => oldText = e.ToString ();
+
+			Application.Top.Add (tf);
+			Application.Begin (Application.Top);
+
+			Assert.Equal ("-1", tf.Text);
+
+			// InsertText
+			tf.SelectedStart = 1;
+			tf.CursorPosition = 2;
+			Assert.Equal (1, tf.SelectedLength);
+			Assert.Equal ("1", tf.SelectedText);
+			Assert.True (tf.ProcessKey (new KeyEvent (Key.D2, new KeyModifiers ())));
+			Assert.Equal ("-2", newText);
+			Assert.Equal ("-1", oldText);
+			Assert.Equal ("-2", tf.Text);
+
+			// DeleteCharLeft
+			tf.SelectedStart = 1;
+			tf.CursorPosition = 2;
+			Assert.Equal (1, tf.SelectedLength);
+			Assert.Equal ("2", tf.SelectedText);
+			Assert.True (tf.ProcessKey (new KeyEvent (Key.Backspace, new KeyModifiers ())));
+			Assert.Equal ("-", newText);
+			Assert.Equal ("-2", oldText);
+			Assert.Equal ("-", tf.Text);
+
+			// DeleteCharRight
+			tf.Text = "-1";
+			tf.SelectedStart = 1;
+			tf.CursorPosition = 2;
+			Assert.Equal (1, tf.SelectedLength);
+			Assert.Equal ("1", tf.SelectedText);
+			Assert.True (tf.ProcessKey (new KeyEvent (Key.DeleteChar, new KeyModifiers ())));
+			Assert.Equal ("-", newText);
+			Assert.Equal ("-1", oldText);
+			Assert.Equal ("-", tf.Text);
+
+			// Cut
+			tf.Text = "-1";
+			tf.SelectedStart = 1;
+			tf.CursorPosition = 2;
+			Assert.Equal (1, tf.SelectedLength);
+			Assert.Equal ("1", tf.SelectedText);
+			Assert.True (tf.ProcessKey (new KeyEvent (Key.X | Key.CtrlMask, new KeyModifiers ())));
+			Assert.Equal ("-", newText);
+			Assert.Equal ("-1", oldText);
+			Assert.Equal ("-", tf.Text);
+		}
 	}
 }