Add warning to widget namespace import.

IPython/html/widgets/__init__.py

@@ -1,24 +1,29 b''
 from .widget import Widget, DOMWidget, CallbackDispatcher, register
 
 from .widget_bool import Checkbox, ToggleButton
 from .widget_button import Button
 from .widget_box import Box, Popup, FlexBox, HBox, VBox
 from .widget_float import FloatText, BoundedFloatText, FloatSlider, FloatProgress, FloatRangeSlider
 from .widget_image import Image
 from .widget_int import IntText, BoundedIntText, IntSlider, IntProgress, IntRangeSlider
 from .widget_output import Output
 from .widget_selection import RadioButtons, ToggleButtons, Dropdown, Select
 from .widget_selectioncontainer import Tab, Accordion
 from .widget_string import HTML, Latex, Text, Textarea
 from .interaction import interact, interactive, fixed, interact_manual
 
 # Deprecated classes
 from .widget_bool import CheckboxWidget, ToggleButtonWidget
 from .widget_button import ButtonWidget
 from .widget_box import ContainerWidget, PopupWidget
 from .widget_float import FloatTextWidget, BoundedFloatTextWidget, FloatSliderWidget, FloatProgressWidget
 from .widget_image import ImageWidget
 from .widget_int import IntTextWidget, BoundedIntTextWidget, IntSliderWidget, IntProgressWidget
 from .widget_selection import RadioButtonsWidget, ToggleButtonsWidget, DropdownWidget, SelectWidget
 from .widget_selectioncontainer import TabWidget, AccordionWidget
 from .widget_string import HTMLWidget, LatexWidget, TextWidget, TextareaWidget
+
+# Warn on import
+from IPython.utils.warn import warn
+warn("""The widget API is still considered experimental and 
+    may change by the next major release of IPython.""")