Listener removes itself from Signal._link

[heuer]

On removal Listener removes

• itself from Signal._link
• sets the reference to the signal to None
• sets the reference to the notify callback to None

Added test case for the issue.

Closes #61

Fixed Signal constructor with no pointer:

• self._ptr uses wl_signal instead of wl_listener
• Added wl_signal_init to ffibuild.

Closes #62