Set CSS manually in the context menu handler

In Chrome apps (and, I assume, other secure pages), Ace triggers the content security policy against unsafe-eval when it uses the `style.cssText` parameter. Most of the time, I don't think this matters, but in the case of the textinput context menu, the error prevents the correct context menu from opening up with copy/paste/etc. I think this is a new change in Chrome 34, because it always worked before. Thanks Google!

This pull request sets the styles manually, which should be safe cross-browser for zIndex and opacity at this point. It's a little more clumsy than the `cssText` solution, but it lets context menus open up in Chrome apps again.