Introduce dedicated Explorer pane for stable Spring outline

[odrotbohm]

The latest Spring Tools releases for VSCode create a dedicated, Spring-specific node in the Outline pane of the Explorer view. That outline is connected to the currently selected element in other Explorer panes / the editor currently active. It would be nice, if a stable, project-scoped variant of that outline would be available as a dedicated pane in the Explorer.

[martinlippert]

Related:

• [structure view] create project structure view UI for VSCode #1492
• [structure view] create project structure view UI for Eclipse #1493