Shouldn't add a new executable if the dialog is canceled

[jcalado]

When canceling the "add executable" openfiledialog, we should not add an empty executable.

[rrebuffo]

The intention behind that is to allow custom and relative paths without using the OpenFileDialog. I think the dialog should not be displayed before adding. Showing that dialog before adding a new Executable is a way of avoiding extra steps but gets in the way if you just need to add many exes in a single relative folder.