Fix inline text field position

Since this commit: https://github.com/sproutcore/sproutcore/commit/4d199900fcd8a60f757c10a82de01b5f0576a907 (which were welcome since besides fixing the z-index, it also fix a problem where if an inline text field was in a scroll view, the field wasn't moving when scrolling), the calculation of position of the editor was completely wrong. This can be see in the showcase, with the SC.LabelView and the SC.ListView examples. Based on my app and the showcase app, this commit fix the problem. I haven't added units test because I believe this can't be tested. Note that I've also dropped IE7 support for this since I'm not able to test it and because even Twitter Bootstrap will drop IE7 support in there next release.