Cleanup config flow (#2932)

* Break up config flow dialog

* Allow picking devices when config flow finishes

* Lint

* Tweaks

src/dialogs/config-flow/step-flow-loading.ts

@@ -0,0 +1,35 @@
+import {
+  LitElement,
+  TemplateResult,
+  html,
+  css,
+  customElement,
+  CSSResult,
+} from "lit-element";
+import "@polymer/paper-spinner/paper-spinner-lite";
+
+@customElement("step-flow-loading")
+class StepFlowLoading extends LitElement {
+  protected render(): TemplateResult | void {
+    return html`
+      <div class="init-spinner">
+        <paper-spinner-lite active></paper-spinner-lite>
+      </div>
+    `;
+  }
+
+  static get styles(): CSSResult {
+    return css`
+      .init-spinner {
+        padding: 50px 100px;
+        text-align: center;
+      }
+    `;
+  }
+}
+
+declare global {
+  interface HTMLElementTagNameMap {
+    "step-flow-loading": StepFlowLoading;
+  }
+}