Restructure frontend code to allow for extensions (#4721)

# Description of Changes
Move frontend code into `core` folder and add infrastructure for
`proprietary` folder to include premium, non-OSS features

frontend/src/core/tools/OverlayPdfs.tsx

@@ -0,0 +1,60 @@
+import { useTranslation } from 'react-i18next';
+import { createToolFlow } from '@app/components/tools/shared/createToolFlow';
+import { useBaseTool } from '@app/hooks/tools/shared/useBaseTool';
+import { BaseToolProps, ToolComponent } from '@app/types/tool';
+import OverlayPdfsSettings from '@app/components/tools/overlayPdfs/OverlayPdfsSettings';
+import { useOverlayPdfsParameters } from '@app/hooks/tools/overlayPdfs/useOverlayPdfsParameters';
+import { useOverlayPdfsOperation } from '@app/hooks/tools/overlayPdfs/useOverlayPdfsOperation';
+import { useOverlayPdfsTips } from '@app/components/tooltips/useOverlayPdfsTips';
+
+const OverlayPdfs = (props: BaseToolProps) => {
+  const { t } = useTranslation();
+
+  const base = useBaseTool(
+    'overlay-pdfs',
+    useOverlayPdfsParameters,
+    useOverlayPdfsOperation,
+    props
+  );
+  const overlayTips = useOverlayPdfsTips();
+
+  return createToolFlow({
+    files: {
+      selectedFiles: base.selectedFiles,
+      isCollapsed: base.hasResults,
+    },
+    steps: [
+      {
+        title: t('overlay-pdfs.settings.title', 'Settings'),
+        isCollapsed: base.settingsCollapsed,
+        onCollapsedClick: base.settingsCollapsed ? base.handleSettingsReset : undefined,
+        tooltip: overlayTips,
+        content: (
+          <OverlayPdfsSettings
+            parameters={base.params.parameters}
+            onParameterChange={base.params.updateParameter}
+            disabled={base.endpointLoading}
+          />
+        ),
+      },
+    ],
+    executeButton: {
+      text: t('overlay-pdfs.submit', 'Overlay and Review'),
+      isVisible: !base.hasResults,
+      loadingText: t('loading'),
+      onClick: base.handleExecute,
+      disabled: !base.params.validateParameters() || !base.hasFiles || !base.endpointEnabled,
+    },
+    review: {
+      isVisible: base.hasResults,
+      operation: base.operation,
+      title: t('overlay-pdfs.results.title', 'Overlay Results'),
+      onFileClick: base.handleThumbnailClick,
+      onUndo: base.handleUndo,
+    },
+  });
+};
+
+export default OverlayPdfs as ToolComponent;
+
+