Delay for UI change before training model (#384)

I think this is worth it for now.
I'd like to revisit moving the work off the UI thread in the longer term.

Author: microbit-robert

src/store.ts

@@ -498,6 +498,9 @@ const createMlStore = (logging: Logging) => {
               trainModelDialogStage: TrainModelDialogStage.TrainingInProgress,
               trainModelProgress: 0,
             });
+            // Delay so we get UI change before training starts. The initial part of training
+            // can block the UI. 50 ms is not sufficient, so use 100 for now.
+            await new Promise((res) => setTimeout(res, 100));
             const trainingResult = await trainModel({
               data: gestures,
               onProgress: (trainModelProgress) =>