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/lms/widgets/HelloWorld.html

@@ -0,0 +1,14 @@
+{#
+  Widget to demonostrate how to write a widget.
+
+  A wiget is a reusable template, that can be used in
+  other templates.
+
+  To this widget can be called as:
+
+    {{ widgets.HelloWorld(name="World") }}
+#}
+
+<div class="hello">
+  Hello, <em>{{ name }}</em>!
+</div>