ifcviewer: load rdb/ifc as property data source on sidecar hit

Sidecar hits skipped opening the underlying .rdb/.ifc, so ifcFile() was
null and the property panel only showed cached name/type/guid. Now, after
a sidecar hit, a background thread opens <stem>.rdb (preferred) or
<stem>.ifc and hands the file to GeometryStreamer via setIfcFile(), with
a dataSourceReady signal so the UI refreshes the current selection.

Gated behind a new AppSettings::loadDataSource toggle (default on) so
users can opt into geometry-only viewing; when off, the sidecar-hit
thread is skipped and the stream-path ifc_file_ is released after
the sidecar write completes.

Also adds *.ifcview to the Add Files dialog filter so a cache can be
opened directly without its source file present.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ifcviewer-full/MainWindow.h

@@ -59,6 +59,7 @@ private slots:
                                 std::vector<PackedElementInfo> elements,
                                 std::string string_table);
     void onLoadedFromSidecar(uint32_t mid, qint64 elapsed_ms);
+    void onDataSourceReady(uint32_t mid);
     void onStreamedElementsReady(uint32_t mid, std::vector<ElementInfo> elements);
     void onLoadedFromStream(uint32_t mid, qint64 elapsed_ms);
     void onLoadCancelled(uint32_t mid);