[RFC] Only pull off two pixels from the window if the taskbar is actually set to auto-hide

Previously we have always added a one pixel gap between the monitor and the window, because the task bar, if set to auto-hide, would not appear at the bottom when hovering with the mouse. This was also mentioned in #567.

This PR only sets the gap if the taskbar is set to auto-hide and also accounts for a taskbar that is not attached to the bottom of the screen, but on any other side.

I used parts of the code from [this blog post](http://codekong.wordpress.com/2010/11/10/custom-window-style-and-accounting-for-the-taskbar/), so I'm not really sure about the license things.

I tested this PR on Windows 8 with a 3-monitor box