The dashed line no longer flickers during resizing.

In the empty view a dashed line is drawn to show that the view can accept drag and drop commands. When resizing the view the origin of the dashed line moves around as Cocoa tries to round from view coordinates to screen pixels. The end result is the dashed like jitters as the view is resized. This may be a retina display only issue.

In this pull request I simple added a property `boundsFromNib` which saves the bounds of the view when it is loaded from the Nib. In the `-drawRect:` all drawing is done with respect to `boundsFromNib` rather than from the rect passed to the method.