New: add layouts

dashboards.layouts.yml

+layout_1:
+  label: 'BC three column'
+  path: templates/layouts
+  template: layouts--1
+  category: 'Dashboards'
+  icon_map:
+    - [one, three]
+    - [two, three]
+  library: dashboards/layouts
+  regions:
+    one:
+      label: Left Top
+    two:
+      label: Left Bottom
+    three:
+      label: Right

templates/layouts/layouts--1.html.twig

+<section {{ attributes }}>
+  <div class="row">
+    <div {{ region_attributes.one }}>
+      {{ content.one }}
+    </div>
+    <div {{ region_attributes.two }}>
+      {{ content.two }}
+    </div>
+    <div {{ region_attributes.three }}>
+      {{ content.three }}
+    </div>
+  </div>
+</section>