ThemeEditor: Store tab title in PropertyTabs as StringView

Removes a hop that creates a deprecated string.

We cannot store the title directly as a String, because we would have to
find a way to propagate the errors during constructing global static
objects.

Userland/Applications/ThemeEditor/MainWidget.h

@@ -72,7 +72,7 @@ struct PropertyGroup {
 };
 
 struct PropertyTab {
-    DeprecatedString title;
+    StringView title;
     Vector<PropertyGroup> property_groups;
 };