Re-use the Mixer's PluginManager (singleton)

Re-use the Mixer's PluginManager (singleton)

Previously there were three plugin-manager instances (two of which
were never destroyed).

There were also 3 sets of context-menus (favorites, by tag, by author..),
all of which were re-built when plugin stati changed.

Since the Plugin-manager is a modal dialog, it can be re-used in
all contexts (route-ui, monitor-section, mixer).