real cancel for insertRef dialog

[octaeder]

This PR resolves #3230. Cancel button now quits the dialog without any effect.
First entry in the list of labels is a surrogate that allows inserting \ref{} (empty command argument) into the document.

Note: When the dialog is presented you can immediately (at least on windows)

• scroll through the list with the up/down arrow keys
• open the list with Alt+down arrow key (image to the right)
• insert \ref{} by pressing Enter or clicking OK button

[sunderme]

Could we change it to <empty> ?

[sunderme]

Thanks