Merge pull request #1422 from rpl/fix/devtools-destroy

Fix dev/pane destroy and unregistering r=@gozala
prototype remote inspectTarget RDP actor

- temporary hack to register custom protocol.js actor created from an addon-sdk addon
- prototyped InspectTarget class
- use InspectTarget in the "dev/pane" addon-sdk module
- update "examples/dev-pane" example to test the prototype