Toggle the terminal by scrolling on the top panel.

This pull request adds the possibility to toggle the terminal by scrolling on the top bar.

I don't know if this is a desired feature for this extension, but in my opinion it enhances the usability quite a bit, especially when using the mouse. 

If it doesn't fit the usecase of the extension, feel free to ignore this PR. Otherwise I hope I didn't miss anything in this implementation, if so I'll fix it up in further commits.