Drop GsPluginJobListInstalledApps in favour of GsPluginJobListApps

Their implementations were pretty similar. GsPluginJobListInstalledApps was added in 42 near the start of this refactoring work; GsPluginJobListApps came later (it hasn’t been in a release yet), and was based on the ListInstalledApps code, learning lessons from it.

The duplication isn’t necessary: this MR adds a GsAppQuery:is-installed tristate property, and callers can use that with GsPluginJobListApps to list all installed apps.

This should save on code, consolidate the query interface a bit more, and be functionally equivalent to before.

This does break API by dropping GsPluginJobListInstalledApps, which was first released in 42. We’ve already bumped the plugin API version this cycle (in commit 10962d70), so that should be OK.

Helps: #1472