Provide a "Silent Install" mode

[alopepeo]

When you have time installing styles, sometimes, open the editor every time you install an style results a bit annoying.

The idea is add an option to the menulist from the USM Toolbar button (like you can enable/disable styles from here) and it should be disabled by default if you want, because open the editor should be useful for the people that is starting with User Styles.

With this option enabled, there is no prompt when you install an style, but you know that the style is installed because Userstyles.org shows the "Style Installed" dialog.

Thanks.