PIM-765 Created the Widget Registry

```
Bug fix: no
Feature addition: yes
Backwards compatibility break: no
Unit test passes: yes
Specs passes: no
Replaced unit test classes removed: not needed
Behat scenario passes: ?
Checkstyle issues: ?
ChangeLog updated: no
Documentation PR: needed (pim_catalog.widget tag)
Fixes the following jira:
 - PIM-765
```