[dashboard] refactor EditorMenu to AddPanelButton (elastic#208226)

PR refactors `EditorMenu` component into `AddPanelButton` to provide
better naming scheme and simplify logic.
* replace `COMMON_VISUALIZATION_GROUPING` with
`ADD_PANEL_VISUALIZATION_GROUP`. Moved from visualizations plugin to
embeddable plugin.
* rename `EditorMenu` => `AddPanelButton`
* rename `DashboardPanelSelectionListFlyout` => `AddPanelFlyout`
* remove unused style `dshSolutionToolbar__editorContextMenu` from
editor_menu.scss
* Simplify loading of `AddPanelFlyout` contents. Replaced
`useGetDashboardPanels` hook with `getMenuItemGroups` function.
* avoid loading `AddPanelFlyout` contents until `AddPanelButton` is
clicked. `DashboardEditingToolbar` component used to setup
`useGetDashboardPanels` hook. Now `AddPanelFlyout` loads contents by
directly calling `getMenuItemGroups`.
* Added loading state to `AddPanelFlyout`
<img width="600" alt="Screenshot 2025-01-24 at 9 21 05 AM"
src="https://github.com/user-attachments/assets/b2dfde5f-e347-4745-9ee0-b3268e2138c2"
/>

---------

Co-authored-by: kibanamachine <[email protected]>
Co-authored-by: Elastic Machine <[email protected]>
Co-authored-by: Hannah Mudge <[email protected]>

src/platform/plugins/shared/dashboard/public/dashboard_app/top_nav/add_new_panel/index.ts → src/platform/plugins/private/links/public/embeddable/embeddable_module.ts

@@ -7,5 +7,5 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
-export { useGetDashboardPanels } from './use_get_dashboard_panels';
-export { DashboardPanelSelectionListFlyout } from './dashboard_panel_selection_flyout';
+export { getLinksEmbeddableFactory } from './links_embeddable';
+export { deserializeLinksSavedObject } from '../lib/deserialize_from_library';