[LANGUAGE] Add clear to turtle (hedyorg#3992)

Fixes hedyorg#3991.
Fun example to test in level 9:
```python
print "Lets Draw!"

repeat 30 times
    if w is pressed
        forward 20
    if a is pressed
        turn -90
    if d is pressed
        turn 90
    if x is pressed
        clear
        print "Cleared!"
```

When using the _clear_ keyword it should now also clear the turtle canvas.

hedy.py

@@ -1709,7 +1709,15 @@ def error_print_nq(self, meta, args):
         return ConvertToPython_2.print(self, meta, args)
 
     def clear(self, meta, args):
-        return 'extensions.clear()'
+        return f"""extensions.clear()
+try:
+    # If turtle is being used, reset canvas
+    t.hideturtle()
+    turtle.resetscreen()
+    t.left(90)
+    t.showturtle()
+except NameError:
+    pass"""
 
 
 @v_args(meta=True)

tests/test_level/test_level_04.py

@@ -638,5 +638,15 @@ def test_lonely_text(self):
 
     def test_clear(self):
         code = "clear"
-        expected = "extensions.clear()"
+        expected = textwrap.dedent("""\
+        extensions.clear()
+        try:
+            # If turtle is being used, reset canvas
+            t.hideturtle()
+            turtle.resetscreen()
+            t.left(90)
+            t.showturtle()
+        except NameError:
+            pass""")
+
         self.multi_level_tester(code=code, expected=expected)