VST3: implement IComponentHandler3

[atsushieno]

It involves createContextMenu() which is UI bound.

[atsushieno]

Floe VST3 queries it.

[atsushieno]

It turned out that we did not even implement IComponentHandler functions.

It is currently part of HostApplication, but we should implement in at instance level (e.g. its performEdit() is instance specific).

[atsushieno]

IComponentHandler ^ is implemented.