Define default 'Generic' module for plugins to be overridden in child classes
Remove duplicate code from placeholder_bar.html using draggable_menu.html
Create a dict-based struct to pass plugins for rendering the lists in the toolbar. Unify similar codes across the codebase.
Add tests for unified plugin struct and for PR #2182
Add documentation and changelog for `plugin_labels`, `plugin_modules`
Merge pull request #2185 from yakky/feature/default_module

Default module and other plugin customization

Fixes #1169