[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/CONTRIBUTORS.rst

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+* Alexey Pelykh <alexey.pelykh@brainbeanapps.com>

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.

web_widget_dropdown_dynamic/readme/ROADMAP.rst

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+ * In v13, ``$.when`` is going to become `Promise.resolve`

web_widget_dropdown_dynamic/readme/USAGE.rst

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+.. code-block:: python
+
+    @api.model
+    def method_name(self):
+        values = [
+            ('value_a', 'Title A'),
+        ]
+        if self.env.context.get('depending_on') == True:
+            values += [
+                ('value_b', 'Title B'),
+            ]
+        return values
+
+.. code-block:: xml
+
+    <field
+        name="other_field"
+    />
+    <field
+        name="char_field"
+        widget="dynamic_dropdown"
+        values="method_name"
+        context="{'depending_on': other_field}"
+    />