Create basic gallery class that finds Things that want to show data

src/openflexure_microscope_server/things/__init__.py

@@ -15,6 +15,13 @@ class OFMThing(lt.Thing):
 
     _data_dir: Optional[str] = None
 
+    _show_in_gallery: bool = False
+
+    @property
+    def show_in_gallery(self) -> bool:
+        """Whether to show in the Gallery."""
+        return self._show_in_gallery
+
     def __enter__(self) -> Self:
         """Set the data directory when the Thing is entered."""
         # Note that the `application_config` was already validated when the