Fix Flatpak delayed loading

The recent patch to allow delayed view loading (0faaeea7) could in some circumstances result in empty views. This was the case in Flatpak builds. If the Grilo Tracker plugin is ready before any of the views get created, the views never receive a 'ready' signal to trigger self population. Add an extra check to populate at the end of BaseView init and do not just rely on the 'ready' signal. This is a workaround.

This patch lays bare some ordering issues and circular logic in the Music startup sequence that needs to be resolved at some point. Added an extended comment in the code for clarification.