Link URI re: issue 486

The proposed changes address the silent failing of adding a link to an unrecognized URI. They allow the user to correct any potential mistakes made to the initial address through a prompt dialogue. 
wouldn't it be better to do
```
<vbox>
  <hbox>
    <label/>
    <textbox/>
  </hbox>
  <hbox>
    <label/>
    <textbox/>
  </hbox>
</vbox>
```
To avoid any possible misalignment (though `pack` _should_ generally take care of that here). It just seems somewhat odd to separate labels from input.