refactor(plugins): improve extensions dialog UI (openscd#538)

Author: JakobVogelsang

src/Plugging.ts

@@ -451,11 +451,17 @@ export function Plugging<TBase extends new (...args: any[]) => EditingElement>(
           >
           </mwc-button>
           <mwc-button
-            raised
+            slot="secondaryAction"
+            icon=""
+            label="${translate('close')}"
+            dialogAction="close"
+          ></mwc-button>
+          <mwc-button
+            outlined
             trailingIcon
             slot="primaryAction"
             icon="library_add"
-            label="${translate('add')}&hellip;"
+            label="${translate('plugins.add.heading')}&hellip;"
             @click=${() => this.pluginDownloadUI.show()}
           >
           </mwc-button>

test/integration/__snapshots__/open-scd.test.snap.js

@@ -875,10 +875,17 @@ snapshots["open-scd looks like its snapshot"] =
     style="--mdc-theme-primary: var(--mdc-theme-error)"
   >
   </mwc-button>
+  <mwc-button
+    dialogaction="close"
+    icon=""
+    label="Close"
+    slot="secondaryAction"
+  >
+  </mwc-button>
   <mwc-button
     icon="library_add"
-    label="Add…"
-    raised=""
+    label="Add custom extension…"
+    outlined=""
     slot="primaryAction"
     trailingicon=""
   >