Weird behaviour on Opera 15

On Opera 15, if I edit a style from Stylish popup menu, the editor is opened in new tab and popup panel.

![stylish-opera15-editstyle](https://f.cloud.github.com/assets/42596/751428/d51066de-e4f7-11e2-9501-7808f955c52d.png)

Also, when I clicked *Managed installed styles* from Stylish popup menu, the manage window is opened in a new window instead of tab. If I remove `target` attribute in popup.html, manage window is opened in popup panel like the above.

This PR fixes this issue.
Tested and worked on Opera 15 and Chrome 28.