Rework AGL App

Closing as no more work will be done to applaunchd for Marlin and all the sub-tasks are finished except for the one about unifying the Qt demo homescreen + launcher applications.  That one I have moved out of the epic as it was not directly related to applaunchd implementation and it was more of a proposed maintenance/performance optimization.  The planned work to switch to using systemd user units in applaunchd and implementing a grpc-based API will be tracked in a new epic / issues.