Necessary changes and fixes to make synth plugins

[vsariola]

Here's a couple of changes that were necessary to compile synthesizer plugins with the vst2 package (to resolve #61):

1. The events field in the structure Events in vst.h was defined wrong; it was void** events where as it should have been void* events[]. In other words: it should be a variable size structure, with some "header", followed by a list of pointers, and should be allocated with a single calloc (and freed with a single free).

2. The user has now mechanism to define arbitrary PluginFlags; specifically, to enable defining vst2.PluginIsSynth flag for the synth

3. I also gave the user a way to define a callback function for responding to PlugCanDo opcode. In my case, I use it to respond positively to vst2.PluginCanReceiveEvents, vst2.PluginCanReceiveMIDIEvent, and vst2.PluginCanReceiveTimeInfo

4. Finally, I gave the user a way to define a callback function for responding to PlugProcessEvents opcodes, to receive MIDI events from the host.

[dudk]

Thanks a lot for your contributions @vsariola! I'll push a tag for the new release later this week. Do you think any other changes would be needed before then?

[vsariola]

Not entirely sure. I think it would be cool to be able to respond to PlugEditOpen and PlugEditClose opcodes, but that's generic thing for plugins. At the moment, I just open my own windows.

Also, I am not responding to very complex events so far, just midi note on and off messages, so there might be something missing there.