Implement support for addon manifest translation.

Implement support for addon manifest translation.

* Addon manifest translations must be kept on <addon>\locale\<lang>\manifest.ini
* Only description and long description are supported.
* AddonManifest can receive an optional argument with a file-like object for the translation data (Addon and AddonBundle must provide it since retrieval is diferent).
* AddonManifest has a new _ methodto retrieve translated data. For instance manifest._('description')
* Other minor code tweaks and cleanups.