add fullscree toggle to home page

lib/sheet_viewer_page.dart

@@ -68,6 +68,9 @@ class _SheetViewerPageState extends State<SheetViewerPage>
 
       document = await PdfDocument.openFile(file.path);
     }
+    setState(() {
+      // document changed
+    });
     return true;
   }
 
@@ -97,6 +100,17 @@ class _SheetViewerPageState extends State<SheetViewerPage>
     return AppBar(
       title: Text(widget.sheet.name),
       actions: [
+        IconButton(
+          icon: Icon(
+            widget.config.fullscreen ? Icons.fullscreen_exit : Icons.fullscreen,
+          ),
+          tooltip: widget.config.fullscreen
+              ? 'Exit Fullscreen'
+              : 'Enter Fullscreen',
+          onPressed: () {
+            toggleFullscreen();
+          },
+        ),
         IconButton(
           onPressed: () {
             setState(() {