Add custom loading widget to PdfPreview widget (#774)

* add custom loading widget to PdfPreview widget

* bump version to 5.5.0

 # Changelog
 
+## 5.5.0
+
+- Add custom loading widget to PdfPreview widget
+
 ## 5.4.3
 
 - Update Pdfium libraries

@@ -59,6 +59,7 @@ class PdfPreview extends StatefulWidget {
     this.previewPageMargin,
     this.padding,
     this.shouldRepaint = false,
+    this.loadingWidget,
   }) : super(key: key);
 
   static const _defaultPageFormats = <String, PdfPageFormat>{
@@ -150,6 +151,10 @@ class PdfPreview extends StatefulWidget {
   /// Force repainting the PDF document
   final bool shouldRepaint;
 
+  /// Custom loading widget to use that is shown while PDF is being generated.
+  /// If null, a [CircularProgressIndicator] is used instead.
+  final Widget? loadingWidget;
+
   @override
   _PdfPreviewState createState() => _PdfPreviewState();
 }
@@ -287,7 +292,10 @@ class _PdfPreviewState extends State<PdfPreview> with PdfPreviewRaster {
     }
 
     if (pages.isEmpty) {
-      return const Center(child: CircularProgressIndicator());
+      return widget.loadingWidget ??
+          const Center(
+            child: CircularProgressIndicator(),
+          );
     }
 
     return ListView.builder(

@@ -7,7 +7,7 @@ description: >
 homepage: https://github.com/DavBfr/dart_pdf/tree/master/printing
 repository: https://github.com/DavBfr/dart_pdf
 issue_tracker: https://github.com/DavBfr/dart_pdf/issues
-version: 5.4.3
+version: 5.5.0
 
 environment:
   sdk: ">=2.12.0 <3.0.0"