consolidate dashboard "add panel" resources into single location

[nreese]

Dashboard "add panel" resources are scatter in multiple places

• ADD_PANEL_TRIGGER is defined in packages/kbn-ui-actions-browser
• COMMON_EMBEDDABLE_GROUPING is defined in src/plugins/embeddable plugin. This does not make sense, since its never used in the embeddable plugin and is not coupled to embeddables
• DashboardPanelSelectionListFlyout is defined in src/plugins/dashboard.
• Dashboard panel placement registry is defined in src/plugins/dashboard.

It would be easier to maintain and document if all of these resources are co-located in a standalone plugin or package.

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)