Use custom app bar for call windows

lib/ui/screens/call_screen.dart

@@ -32,10 +32,15 @@ class CallScreen extends StatefulWidget {
   /// This settings should be always true except for the small call window...
   final bool floatingButtons;
 
+  /// Use custom application bar. This function takes as parameter a nullable
+  /// String which is the title of the conversation
+  final PreferredSizeWidget Function(String) buildCustomAppBar;
+
   const CallScreen({
     Key key,
     @required this.convID,
     this.floatingButtons = true,
+    this.buildCustomAppBar,
   })  : assert(convID != null),
         assert(convID > 0),
         assert(floatingButtons != null),
@@ -453,22 +458,26 @@ class _CallScreenState extends SafeState<CallScreen> {
     return GestureDetector(
       onDoubleTap: () => _toggleMenuBarsVisibility(),
       child: Scaffold(
-        appBar: _canHideMenuBar
-            ? null
-            : AppBar(
-                leading: IconButton(
-                  icon: Icon(Icons.arrow_back),
-                  onPressed: () => _leaveCall(),
-                ),
-                title: _convName == null
-                    ? CircularProgressIndicator()
-                    : Text(_convName),
-              ),
+        appBar: _canHideMenuBar ? null : _buildAppBar(),
         body: _buildBody(),
       ),
     );
   }
 
+  /// Build application bar
+  PreferredSizeWidget _buildAppBar() {
+    if (widget.buildCustomAppBar != null)
+      return widget.buildCustomAppBar(_convName);
+
+    return AppBar(
+      leading: IconButton(
+        icon: Icon(Icons.arrow_back),
+        onPressed: () => _leaveCall(),
+      ),
+      title: _convName == null ? CircularProgressIndicator() : Text(_convName),
+    );
+  }
+
   /// Build widget body
   Widget _buildBody() {
     // Handle errors