Barcode scanner updates (#562)

* Add BUILDING.md

* Replace scaning library

- Out with qr_code_scanner
- In with flutter_zxing

* Update specs for jdk / kotlin / gradle

- NFI what this all means?

* Refactor barcode scanning widget

* Refactor barcode overlay

* Add handlers

* Update release notes

* Fix AppBar color

* Enhance attachment widget

* remove unused import

* Improved icon

* Select theme from main drawer

lib/widget/attachment_widget.dart

@@ -208,11 +208,8 @@ class _AttachmentWidgetState extends RefreshableState<AttachmentWidget> {
 
     if (tiles.isEmpty) {
       tiles.add(ListTile(
+        leading: Icon(TablerIcons.file_x, color: COLOR_WARNING),
         title: Text(L10().attachmentNone),
-        subtitle: Text(
-          L10().attachmentNoneDetail,
-          style: TextStyle(fontStyle: FontStyle.italic),
-        ),
       ));
     }