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.

frontend/ui/plugin/background_task_plugin.lua

@@ -26,6 +26,9 @@ function BackgroundTaskPlugin:_schedule(settings_id)
         repeated = enabled,
         executable = self.executable,
     })
+    local Event = require("ui/event")
+    local UIManager = require("ui/uimanager")
+    UIManager:broadcastEvent(Event:new("BackgroundJobsUpdated"))
 end
 
 function BackgroundTaskPlugin:_start()