Add two-way string port support.
Add two-way string port support.

Send configure message via OSC when hosts calls port_event on a string port.
Also, the string port flag is now cached inside plugin_proxy_base.

zamaudio/calf-LR4,jnetterf/calf,adiknoth/calf,adiknoth/calf,jnetterf/calf,adiknoth/calf,zamaudio/calf-LR4,jnetterf/calf,jnetterf/calf,adiknoth/calf,jnetterf/calf,zamaudio/calf-LR4,zamaudio/calf-LR4,zamaudio/calf-LR4,adiknoth/calf