Fixes scrubbing issues with mouse and window

[sillero]

fixes #516

• 1px margin on the progress bar to detect when the mouse is leaving the window horizontally (effectively no visual impact)
• dynamic vertical threshold to better manage the mouse behavior on scrubbing
• needed to detect the gui.window focus/blur, so I'm dispatching it to jQuery too, which should also be useful in other scenarios

[weblancaster]

Awesome work @sillero thanks a lot and sorry for my delay in reviewing.