fix: make launcher entries expandable

Can't really tell why this broke. At first I thought this was due to
changes of `St.Bin` default expand properties, but `St.BoxLayout`
isn't descendant of it. Setting `x_expand` on the button itself didn't
fix this.

src/search.ts

@@ -429,7 +429,7 @@ export class SearchOption {
         this.exec = exec;
         this.keywords = keywords;
 
-        const layout = new St.BoxLayout({});
+        const layout = new St.BoxLayout({ x_expand: true });
 
         attach_icon(layout, category_icon, icon_size / 2);