Reorganises internal state

`options` slice was huge and managed a mix of generation parameters and general app settings. It has been split up:

- Generation parameters are now in `generationSlice`.
- Postprocessing parameters are now in `postprocessingSlice`
- UI related things are now in `uiSlice`

There is probably more to be done, like `gallerySlice` perhaps should only manage internal gallery state, and not if the gallery is displayed.

Full-slice selectors have been made for each slice.

Other organisational tweaks.

invokeai/frontend/src/features/parameters/components/ProcessButtons/ProcessButtons.tsx

@@ -0,0 +1,22 @@
+import InvokeButton from './InvokeButton';
+import CancelButton from './CancelButton';
+import LoopbackButton from './Loopback';
+import { useAppSelector } from 'app/storeHooks';
+import { activeTabNameSelector } from 'features/ui/store/uiSelectors';
+
+/**
+ * Buttons to start and cancel image generation.
+ */
+const ProcessButtons = () => {
+  const activeTabName = useAppSelector(activeTabNameSelector);
+
+  return (
+    <div className="process-buttons">
+      <InvokeButton />
+      {activeTabName === 'img2img' && <LoopbackButton />}
+      <CancelButton />
+    </div>
+  );
+};
+
+export default ProcessButtons;