[ivette] First steps for external code edition

For now I have the following:
  - A new preference that allows to specify which command to launch to
    edit the source code in the user's favorite editor;
  - Spawn a new process that launch the editor on the current selection
    using the specified command. The keybinding used is
    <Ctrl-LeftClick>, exactly as the old GUI.

However, there is a catch. First, the keybinding does not select a line
before launching the external editor. Second, the only selection that is
actually taken into consideration is the one done when we click on a
function.