Bug 963039 - [B2G][Settings] Developer settings are too hard to access

- Move the developer settings from `Settings > Device Information > More Information > Developer` to `Settings > Developer`.
- Show/hide the developer settings depending on the `Settings > Device Information > More Information > Developer menu` checkbox (pref `developer.menu.enabled`).

FIXME:
- Developer menu shouldn't have the same icon as Accessibility.
- Tests?