Accessibility fixes

- Aria landmarks
 - Title attributes to all icon only buttons
 - <Multibutton/> now internally a radio group
 - Replaced 1 'skip navigation link' with UI group links
 - Added map specific shortcuts to the shortcut menu
 - Hidden layer list actions from tab index

src/styles/_modal.scss

@@ -256,17 +256,33 @@
 }
 
 .maputnik-modal-shortcuts {
-  code {
+  position: relative;
+  overflow: hidden;
+  max-width: 30em;
+
+  kbd {
     color: white;
     background: #3c3c3c;
     padding: 2px 6px;
     display: inline-block;
     text-align: center;
     border-radius: 2px;
-    margin-right: 4px;
     font-family: monospace;
   }
 
+  &__shortcut {
+    margin-bottom: $margin-2;
+  }
+
+  dt {
+    display: inline;
+    margin-right: $margin-2;
+  }
+
+  dd {
+    display: inline;
+  }
+
   li {
     margin-bottom: 4px;
   }