lib: Add new GsPluginJobListCategories job

This replaces the gs_plugin_add_categories() vfunc and the gs_plugin_loader_get_categories_async() method. It combines listing categories with refining them.

Refining a category is a generalisation of the existing gs_plugin_add_categories() vfunc. Despite its name, the existing vfunc only calculates how many apps there are in each category and calls gs_category_increment_size() accordingly. The new refine_categories_async() vfunc will do the same, but should be more easily expandable to refine other properties of a category in future, if needed. It also reuses existing terminology so the API is more consistent and thus a bit easier to learn.

Signed-off-by: Philip Withnall pwithnall@endlessos.org

Helps: #1472