appstream: Properly handle "all" desktop group for a category
The current logic only updates app-counter for parent categories only if there are multiple groups/subcategories. However, this is not correct as it may lead to categories with no groups/ subcategories, be hidden from the overview-page.
This bug was discovered inside Endless, where a custom category with no groups/subcategories was added and it was not ending up in the UI. Pendatically speaking, each parent category has atleast one group/subcategory i.e. "all". In this patch, we correct the handling of "all" and update the app-counter for parent category. Then if the parent cateogory has multiple groups/subcategories, app-counter for those subcategories are updated as well.