Add shadows parameters

Author: martinRenou

docs/source/drawing_shapes.rst

@@ -252,6 +252,37 @@ You can also change the global transparency.
 
 .. image:: images/colored_rect.png
 
+Shadows
++++++++
+
+You can easily draw shadows by tweaking the following attributes:
+
+- ``shadow_offset_x``: (float) Indicates the horizontal distance the shadow should extend from the object. This value isn't affected by the transformation matrix. The default is ``0``.
+- ``shadow_offset_y``: (float) Indicates the vertical distance the shadow should extend from the object. This value isn't affected by the transformation matrix. The default is ``0``.
+- ``shadow_blur``: (float) Indicates the size of the blurring effect; this value doesn't correspond to a number of pixels and is not affected by the current transformation matrix. The default value is ``0``.
+- ``shadow_color``: (valid HTML color) A standard CSS color value indicating the color of the shadow effect; by default, it is fully-transparent black: ``'rgba(0, 0, 0, 0)'``.
+
+.. code:: Python
+
+    from ipycanvas import Canvas
+
+    canvas = Canvas(size=(200, 200))
+
+    canvas.shadow_color = 'green'
+    canvas.shadow_offset_x = 2
+    canvas.shadow_offset_y = 3
+    canvas.shadow_blur = 3
+
+    canvas.fill_rect(25, 25, 100, 100)
+    canvas.clear_rect(45, 45, 60, 60)
+
+    canvas.shadow_color = 'blue'
+    canvas.stroke_rect(50, 50, 50, 50)
+
+    canvas
+
+.. image:: images/shadows.png
+
 Lines styles
 ++++++++++++
 

docs/source/drawing_text.rst

@@ -44,4 +44,4 @@ You can change the text style by changing the following ``Canvas`` attributes:
 - ``direction``: (str) Directionality. Possible values: ``"ltr"``, ``"rtl"``, ``"inherit"``. The default value is ``"inherit"``.
 
 .. note::
-    You can still use ``fill_style`` and ``stroke_style`` for coloring text
+    You can still use ``fill_style``, ``stroke_style``, ``shadow_color`` etc for coloring text and applying shadows.

docs/source/images/shadows.png

@@ -8,7 +8,7 @@
 
 import numpy as np
 
-from traitlets import Enum, Float, Instance, List, Tuple, Unicode, observe
+from traitlets import Enum, Float, Instance, List, Tuple, Unicode
 
 from ipywidgets import CallbackDispatcher, Color, DOMWidget, Image, widget_serialization
 
@@ -71,6 +71,22 @@ class Canvas(DOMWidget):
         default_value='source-over'
     )
 
+    #: (float) Indicates the horizontal distance the shadow should extend from the object.
+    #: This value isn't affected by the transformation matrix. The default is 0.
+    shadow_offset_x = Float(0.0)
+
+    #: (float) Indicates the vertical distance the shadow should extend from the object.
+    #: This value isn't affected by the transformation matrix. The default is 0.
+    shadow_offset_y = Float(0.0)
+
+    #: (float) Indicates the size of the blurring effect; this value doesn't correspond to a number of pixels
+    #: and is not affected by the current transformation matrix. The default value is 0.
+    shadow_blur = Float(0.0)
+
+    #: (valid HTML color) A standard CSS color value indicating the color of the shadow effect; by default,
+    #: it is fully-transparent black.
+    shadow_color = Color('rgba(0, 0, 0, 0)')
+
     #: (float) Sets the width of lines drawn in the future, must be a positive number. Default to ``1.0``.
     line_width = Float(1.0)
 
@@ -428,7 +444,8 @@ def __setattr__(self, name, value):
 
         canvas_attrs = ['fill_style', 'stroke_style', 'global_alpha', 'font', 'text_align',
                         'text_baseline', 'direction', 'global_composite_operation',
-                        'line_width', 'line_cap', 'line_join', 'miter_limit', 'line_dash_offset']
+                        'line_width', 'line_cap', 'line_join', 'miter_limit', 'line_dash_offset',
+                        'shadow_offset_x', 'shadow_offset_y', 'shadow_blur', 'shadow_color']
         if name in canvas_attrs:
             command = {
                 'name': 'set',