Instrument: return true in default implementation of handleMidiEvent()

Supress warnings about unhandled MIDI events e.g. on PianoView focus out
by returning true in Instrument::handleMidiEvent().
Do not pass default time to processInEvent()

We introduced default parameters for MidiTime parameters so there's no
need to construct them manually.
SampleBuffer: define OV_EXCLUDE_STATIC_CALLBACKS

Define OV_EXCLUDE_STATIC_CALLBACKS in order to suppress compiler warnings
about unused static instances when building Win32 version.
RemoteVstPlugin: adapted MIDI changes

Fixes build failure after recent API changes.

Closes #157.