make PluginManager API more in line with other singletons; do initial plugin discover...

make PluginManager API more in line with other singletons; do initial plugin discovery before beginning to construct the UI, so that if plugins create GUIs (e.g for license verification) they don't cause a run loop to catch the UIManager in an inconsistent state with menus defined but actions missing

git-svn-id: svn://localhost/ardour2/branches/3.0@10586 d708f5d6-7413-0410-9779-e7cbd77b26cf