Make ctrl+click add a new cursor in Linux and Windows, fixes #2105

In Chromium the metaKey property of a mousedown event might never get set to true on Linux and Windows as there is no special, well defined meta key on the keyboard (see this [issue](https://code.google.com/p/chromium/issues/detail?id=95874)). Hence I made editor-view.coffee check for e.ctrlKey instead if the platform is not Mac, which makes ctrl+click to add a cursor as desired.