Add build support for methcla pro
Move plugin loading to Environment

Move plugin manager to Environment implementation and add
pluginLibraries option field, making it easier to use Environment
without Engine.
Expose cast operators instead of explicit Environment conversion methods
Pass around pointers to reference counted requests

The message queue and scheduling queue hold onto their items for their
entire lifetime. Passing around pointers avoids leaking memory for
request packets.
Hide more implementation details in EnvironmentImpl
Make Environment::process public
Don't allow negative sample offset for synth
Support for nested bundles and separate synth activation from creation