Improve search appearance

lib/ui/screens/search_screen.dart

@@ -35,6 +35,9 @@ class _SearchScreenState extends State<SearchScreen> {
       child: Column(
         children: <Widget>[
           TextField(
+            decoration: InputDecoration(
+                icon: Icon(Icons.search),
+                labelText: tr("Search a user, a group...")),
             onChanged: (s) => _newSearch(s),
           ),
           _searchResultsList == null