position based on workarea instead of screen

[kufii]

When positioning based off the screen, certain panels (Dash To Dock for example) end up covering the terminal. This change uses the workarea coordinates instead

[bigbn]

@kufii There is still issue exist with "dash to panel" extension when extra unnecessary space is allocated on the top of screen.

[kufii]

For some reason I'm unable to add more commits to this PR, so I'm closing it and creating a new one. I fixed the issue @bigbn reported though.

#211