add callbacks for clipboard access

[ueno]

To enable input-methods to supply text from clipboard, add necessary API to register callbacks, as libkkc does: https://github.com/ueno/libkkc/blob/master/libkkc/context.vala#L201 https://github.com/ueno/libkkc/blob/master/libkkc/context.vala#L208

which could be used like: https://github.com/ueno/ibus-kkc/blob/master/src/engine.vala#L171

Moved from #40.