dev/pane fixes and tweaks

small fixes and tweaks on "dev/pane" module (WIP on mozilla/addon-sdk#1412):

- fix returning instance from build function (fix unregistering devtool panel): e8c0c33
- restrict devtool panel to local targets: dd6a4ec
- unregister devtool panel on destroy and unregister all panels on unload: d9decab
- minor refactor on setting attributes on inner and outer frames: 80df1a3