Add MasterDetailPage.bottomBar

[jpnurmi]

Allows inserting an arbitrary sticky item at the bottom of the master list, similar to the app bar at the top.

Close: #440

[jpnurmi]

For the screenshot attached to #440 I used:

diff --git a/example/lib/example.dart b/example/lib/example.dart
index 074aca1..abaca1a 100644
--- a/example/lib/example.dart
+++ b/example/lib/example.dart
@@ -81,6 +81,22 @@ class _ExampleState extends State<Example> {
             appBar: AppBar(
               title: const Text('Example'),
             ),
+            bottomBar: BottomAppBar(
+              elevation: 0,
+              child: Column(
+                mainAxisSize: MainAxisSize.min,
+                children: const [
+                  Divider(height: 1),
+                  Padding(
+                    padding: EdgeInsets.symmetric(vertical: 8.0),
+                    child: YaruMasterTile(
+                      leading: Icon(YaruIcons.settings),
+                      title: Text('Settings'),
+                    ),
+                  ),
+                ],
+              ),
+            ),
           );
   }
 }

[Feichtmeier]

That's a cool feature - thank you!