Add hang up button

lib/ui/screens/call_screen.dart

@@ -178,7 +178,7 @@ class _CallScreenState extends SafeState<CallScreen> {
     if (_membersList == null) return buildCenteredProgressBar();
 
     return Column(
-      children: <Widget>[_buildMembersArea()],
+      children: <Widget>[_buildMembersArea(), Spacer(), _buildFooterArea()],
     );
   }
 
@@ -200,4 +200,21 @@ class _CallScreenState extends SafeState<CallScreen> {
       ),
     );
   }
+
+  /// Footer area
+  Widget _buildFooterArea() {
+    return Material(
+      color: Colors.black,
+      child: Row(
+        mainAxisAlignment: MainAxisAlignment.center,
+        children: <Widget>[
+          // Hang up call
+          IconButton(
+            icon: Icon(Icons.phone, color: Colors.red),
+            onPressed: () => _leaveCall(),
+          ),
+        ],
+      ),
+    );
+  }
 }