[ADD] web_widget_dropdown_dynamic

[UPD] Update web_widget_dropdown_dynamic.pot

[UPD] Update web_widget_dropdown_dynamic_example.pot

[UPD] README.rst

[ADD] icon.png

[UPD] README.rst

web_widget_dropdown_dynamic/readme/DESCRIPTION.rst

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+Dynamic dropdown widget that supports resolving options from backend of:
+
+ * ``fields.Char``
+ * ``fields.Integer``
+ * ``fields.Selection``
+
+**NOTE:** This widget is not intended to *extend* ``fields.Selection``, but to
+filter selection values. For fully-dynamic set of options, use ``fields.Char``
+instead.