Bug 907450 - Anchored Panels …

- Allows `button` and `widget` to be specified as `position` value for panels.
- added `getFocusedBrowser` to window/utils
- use `getFocusedBrowser` in `state` instead of `getMostRecentBrowserWindow`
- Added test case
Merge pull request #1398 from ZER0/anchored-panel/907450

fix Bug 907450 - Anchored Panels r=@gozala