Add `widget_tag` method

No tests. No docs
Render buffer

No tests
Stateful widgets

No tests. No docs
Use HashWithIndifferentAccess for tree's children hash

No tests. No docs
Merge branch 'stateful' into develop-suitcase
Merge branch 'use-hash_with_indifferent_access-for-children_hash' into develop-suitcase
Merge branch 'widget_tag-method' into develop-suitcase
Output `trigger`s' results in order they appear
Output straight to `page_updates` when `render_buffer`
Fix `trigger`

`page_updates` should be `dup`licated before `fire` because of
everything is a reference in Ruby
Modules lookup is doing when `widget` method creates a widget

No tests, no docs