Merge branch 'update-widgets-ref' into 'master'

...mages/import_documentation/Widgets-API-Developers-OLD.png

...ic/images/import_documentation/Widgets-API-Developers.png

source/import_documentation/Components/Solid-Ac-Checker2.rst

source/import_documentation/Components/Solid-Widget.rst

source/import_documentation/Mixins/counter-mixin.rst

source/import_documentation/Mixins/filter-mixin.rst

source/import_documentation/Mixins/grouper-mixin.rst

source/import_documentation/Widgets/Reference.rst

...rt_documentation/framework_guide/list-base-components.rst

...ocumentation/framework_guide/list-external-components.rst