Dragging interop fix

Most users will not notice any difference, but I've changed the way
dragging windows is handled specifically to allow interop with the the
sticky windows library here:

https://bitbucket.org/alfonse/wpf-window-snap
Merge pull request #434 from 5flags/master

Window drag interop fix
fix for #441 from @dak8
Merge branch 'master' of github.com:MahApps/MahApps.Metro