Added Window to menu bar with Minimize action

By convention, all native OS X apps have a `Window` menu with `Minimize`, `Zoom`, and `Bring All to Front` actions. Additionally, applications make use of the ⌘+M hotkeys for minimizing. This PR is primarily targeted toward enhancing usability for every day Mac users.

To keep the code simple, and for greater interface consistency, I have submitted this as a platform independent feature.

This PR brings in a few additional translations. I'm happy to submit an update with the required `bitcoin_en.ts` updates. `make translate` seems currently broken in master.