Bug 732294 - Panel constructor lacks a way to set position of panel oher than "centered"

- Added `position` in panel's options
- Added `options` as argument for `panel.show` with `width`, `height`, `position` and `focus`
- Fixed `zoom`: removed undefined `mozScreenPixelsPerCSSPixel`, and added `screenPixelsPerCSSPixel` (nsIDOMWindowUtils) to `window/utils` (see Bug 507755)