Share assets from mobile to other apps (#435)

* Share unique assets

* Style share preparing dialog

* Share assets from multiselect

* Fix i18n

* Use navigator like in delete dialog

* Center bottom-bar buttons

mobile/lib/shared/ui/share_dialog.dart

@@ -0,0 +1,23 @@
+import 'package:easy_localization/easy_localization.dart';
+import 'package:flutter/material.dart';
+
+class ShareDialog extends StatelessWidget {
+  const ShareDialog({Key? key}) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return AlertDialog(
+      content: Column(
+        mainAxisSize: MainAxisSize.min,
+        children: [
+          const CircularProgressIndicator(),
+          Container(
+            margin: const EdgeInsets.only(top: 12),
+            child: const Text('share_dialog_preparing')
+                .tr(),
+          )
+        ],
+      ),
+    );
+  }
+}