Signal disconnection and memory management

[mappu]

When a signal is connected to a C++ object, the Go side calls cgo.NewHandle() to allocate the callback. When the C++ object is deleted, there's no way for Go to know that it can GC that memory.

It may be possible to find some way to detect object deletion and free associated resources.

There should also be an explicit way to disconnect a signal, both for a user's own purpose, and for the purpose of freeing memory.