Apply opacity to context-menu

When you click an identifier, it is usual that there is a match nearby. But the context menu could cover that match, so that you can never find it.

By making the context menu translucent, the covered hightlight can be noticed.

![context-menu-translucent](https://cloud.githubusercontent.com/assets/333750/5192582/d713257a-754c-11e4-8521-03a4ad4904aa.png)