Refactor app listing to use a new GsAppQuery class

See the commit messages. The key improvements that this approach provides are:

• Consolidating a lot of similar plugin vfunc implementations into one. In particular, this means the implementation of refining search results, filtering, sorting and max-results can be shared.
• Adding GsAppQuery:filter-func allows callers to filter the results before max-results is applied, which can be used in future to fix a bug with the featured carousel, where the number of search results is truncated, and then the list is filtered for apps with hi-res icons, which can further truncate the results.

This replaces the ACTION_GET_RECENT and ACTION_SEARCH_FILES actions with a new GsPluginJobListApps job and associated vfunc. Together with GsAppQuery (which immutably represents a query), this job can be used to implement both old actions.

More actions could be replaced by GsPluginJobListApps in future, but I’ve not done that in this MR to keep it simple and to validate the concept. If this MR gets accepted I’ll replace these additional actions in another MR:

• GS_PLUGIN_ACTION_GET_POPULAR
• GS_PLUGIN_ACTION_GET_FEATURED
• GS_PLUGIN_ACTION_SEARCH
• GS_PLUGIN_ACTION_SEARCH_PROVIDES
• GS_PLUGIN_ACTION_GET_CATEGORY_APPS
• GS_PLUGIN_ACTION_GET_ALTERNATES

I will probably also remove GsPluginJobListInstalledApps in favour of using GsPluginJobListApps with a new is-installed property in GsAppQuery in a followup MR, if this one is accepted.

I tested that ACTION_GET_RECENT still works by looking at the “New & Updated” sections on the overview and in a category, and verifying that the same apps are listed there as without the branch applied. I tested that ACTION_SEARCH_FILES still works by running:

gdbus call --session --dest org.gnome.Software --object-path /org/gnome/Software --method org.gtk.Actions.Activate '"install-resources"' "[<('install-provide-files', ['/usr/bin/evolution'], 'startup-id', 'desktop-id', 'ident')>]" "@a{sv} {}"

Helps: #1472