[vim] Make vim dialog work correctly

[njx]

Currently, running a vim command like :w doesn't work--it throws an exception when closing the dialog because it tries to call callback, which doesn't exist. If I'm understanding the code correctly, it should call onClose instead.

(It looks like this code has been in there for a long time, so I'm not sure why this hasn't caused a problem before--maybe no one is using the default implementation of openDialog().)

[mightyguava]

You are right, my bad. Thanks for the fix.

[marijnh]

Thank you. Merged.