Bug in multi-monitor positioning code

[CaiusJard]

If one has a screen arrangement such as:

with e.g. monitor 3 being the primary, it means the fullScreen.Left bound is negative

The calc for the horizontal center thus needs to include the left bound as a starting point:

   var centerOfAllScreensX = fullScreen.Left + fullScreen.Width/2.0;

Line 25 of Dragger.cs just has var centerOfAllScreensX = fullScreen.Width/2.0; which I believe only works as intended if the fullscreen bound starts from a Left of 0.