Bind missing interfaces from the sdk

[m-hilgendorf]

Non exhaustive list:

• [x] vst/ivstchannelcontextinfo.h
• [x] vst/ivstcontextmenu.h
• [ ] vst/ivsthostapplication.h
• [ ] vst/ivstinterappaudio.h
• [ ] vst/ivstphysicalui.h
• [ ] vst/ivsttestplugprovider.h
• [ ] vst/vstpresetkeys.h
• [ ] vst/vstpshtpack4.h
• [ ] vst/vstspeaker.h

We also need bindings for the missing constants.

[MirkoCovizzi]

What is vst/ivstplugin? I can't seem to find it in the VST3 source

[m-hilgendorf]

My bad, that's a local file on my machine.