Positioning corrections

Atom 0.126.0 broke the `CoveringView` positioning again:

![screen shot 2014-09-11 at 9 22 25 am](https://cloud.githubusercontent.com/assets/17565/4234414/b0e3a996-39b6-11e4-8c7a-c4dd93d9ce65.png)

Investigation reveals that `EditorView.lineHeight` is `undefined` when an EditorView is detached, which is making `pixelPositionForBufferPosition` always return 0. This is throwing off the positioning calculation, which is problematic.