properties_view: Add comment explaining the purpose of build_dimensions()

Sophie Herold mentioned that it's not entirely clear why we're
loading the image data again here. This comment should explain why
to future readers.

src/widgets/properties_view.rs

@@ -233,6 +233,11 @@ impl LpPropertiesView {
         self.notify("file-info");
     }
 
+    // Determines the dimensions of the file asyncronously. We do this
+    // asycnhronously to avoid blocking the UI as we parse the file info.
+    // In addition, we're loading the image metadata separately from the file
+    // here so that the properties sidebar's information can load separately
+    // from the current file.
     fn build_dimensions(&self, info: &gio::FileInfo, file: &gio::File) {
         let imp = self.imp();