Tame BackgroundRunner: stop running when no more job (#6605)

A BackgroundRunner plugin instance will stop running
(rescheduling a check every 2 seconds) when there is no
(or no more) job to run.
Clients of this service now have to emit an event after
adding a job into PluginShare.backgroundJobs, so an
already loaded but stopped BackgroundRunner can notice
it and start running again.

spec/unit/commonrequire.lua

@@ -103,3 +103,9 @@ stopBackgroundRunner = function()
     background_runner = nil
     require("pluginshare").stopBackgroundRunner = true
 end
+
+notifyBackgroundJobsUpdated = function()
+    if background_runner then
+        background_runner:onBackgroundJobsUpdated()
+    end
+end