Bug 836930 - selection module may have early gc issues for system/event listeners

- Added strong reference to avoid GC-ed earlier
Bug 836930
- Removed unnecessary listener
Bug 836930:
- Added strong reference to `document-shown` and remove it during the unload
- Added unit test for the `document-shown` scenarios with `forceGC`
Merge pull request #761 from ZER0/selection-gc/836930

Fix Bug 836930 - selection module may have early gc issues for system/event listeners r=@erikvold