This commit refactors the interaction between the diagnostics editors
(`BufferDiagnosticsEditor` and `ProjectDiagnosticsEditor`) so that,
instead of passing an optional enum to the `ToolbarControls` struct, as
well as the `DiagnosticBlock`, we pass a reference to a type that
implements a new trait, `DiagnosticsToolbarEditor`, which defines the
methods that the toolbar controls needs, namely:
- `include_warnings`
- `toggle_warnings`
- `has_stale_excerpts`
- `is_updating`
- `stop_updating`
- `refresh_diagnostics`
- `get_diagnostics_for_buffer`
This makes it so that both `ToolbarControls` and `DiagnosticBlock`
implementations don't need to be aware of the internal details of each
of the editors, as well as avoiding duplicating the
`DiagnosticsEditorHandle` enum.
67b3f80003