further improved accessibility across the application and add tooltips to action buttons

lib/widget/file_viewer.dart

@@ -186,6 +186,7 @@ class _FileViewerState extends State<FileViewer> {
       ...actions,
       if (showActionsMenu)
         PopupMenuButton<FileViewingActions>(
+          tooltip: 'Dateiaktionen',
           onSelected: _handleAction,
           itemBuilder: (context) => _availableActions()
               .map(
@@ -282,8 +283,10 @@ class _FileViewerState extends State<FileViewer> {
     ),
   );
 
-  Widget _buildPdfView() =>
-      Scaffold(appBar: _appbar(), body: DeferredPdfViewer(path: widget.path));
+  Widget _buildPdfView() => Scaffold(
+    appBar: _appbar(),
+    body: DeferredPdfViewer(path: widget.path),
+  );
 
   Widget _buildVideoView() => Scaffold(
     appBar: _appbar(),