feat: widgets interface

Widgets are reusable jinja templates which can be used in other
themplates. Widgets are written in widgets/ directory in every frappe
module and can be accessed as `{{ widgets.WidgetName(...) }}` from any
template.

community/test_widgets.py

@@ -0,0 +1,16 @@
+# -*- coding: utf-8 -*-
+# Copyright (c) 2021, FOSS United and Contributors
+# See license.txt
+import frappe
+import unittest
+from .widgets import Widget, Widgets
+
+class TestWidgets(unittest.TestCase):
+    def test_Widgets(self):
+        widgets = Widgets()
+        assert widgets.Foo.name == "Foo"
+        assert widgets.Bar.name == "Bar"
+
+    def _test_Widget(self):
+        hello = Widget("HelloWorld")
+        assert hello(name="Test") == "Hello, Test"