tree: Clicking row should check radio/checkboxes

Addresses #37 (closed)

I am going to let this cook as a merge request for a while before merging.

Since zenity3 is basically legacy software at this point that I had planned to keep in pure maintenance mode, I am reluctant to commit any changes to its behaviour unless we can be absolutely certain that pre-existing scripts from popular applications do not break or that unexpected behaviour does not occur.