Refactor GET_POPULAR to use GsPluginJobListApps

This adds an is-curated property to GsAppQuery and uses that instead of GET_POPULAR/gs_plugin_add_popular().

The changes are fairly straightforward (similar to all the previous job refactoring) apart from:

• There are some additional changes to drop the GNOME_SOFTWARE_POPULAR environment variable, which was used to communicate test data between the dummy unit tests and the dummy plugin. This is now done in a different hacky way, but it’s now at least thread-safe and doesn’t complicate the GsPluginLoader code.
• The terminology has changed from ‘popular’ to ‘curated’, since these apps are curated by an editor rather than from a popular vote or similar.

This follows on from !1328 (merged).

Helps: #1472