Better/smarter Drumstick RT plugins loading and unloading

[pedrolcl]

Right now, all available plugins are loaded at application startup, and they remain initialized until the application ends.

• The plugins should only be loaded and initialized when needed. For instance, the output synthesizers draw power/CPU cycles even when they aren't used. This should be avoided.
• Problems within the plugins (for instance, right now the Windows MIDI input plugin) may avoid application termination. It would be better to unload the plugins explicitly when not needed anymore.