changed the transparency of the window where drag is from on mac.

git-svn-id: http://svn.synergy-foss.org/trunk@1881 33c40d52-04ba-11de-a43f-ff96a2b7a9af
- using cocoa code to fake mouse button down instead of using synergy code.
- fake a ctrl key up and esc key down and up before leave the screen if there is a dragging operation.

git-svn-id: http://svn.synergy-foss.org/trunk@1882 33c40d52-04ba-11de-a43f-ff96a2b7a9af