Diagram Editor: Having multiple palettes for one Diagram is problematic

As an application developer I'd like to avoid problems that occur when multiple - maybe “conflicting” - palettes exist for one overall diagram (with sub-diagrams). There should be the option to have only one palette (no extra palettes for sub diagrams). It should only be in the responsibility of the palette entry whether the corresponding element can be inserted at the drag destination or not. Contributing additional palette entries should be possible using a dedicated API.

Start by locating the Diagram Editor's palette handling and the palette entries mentioned in the issue; no files or tests are identified. Clarify the single-palette behavior for diagrams with sub-diagrams, the insertion decision each palette entry must make, and the scope of the dedicated API for adding entries.

Written by the indexing model from the issue text.