osx: fix annoying focus behaviour (#689)

OS X has a global menu bar and a per app dock icon. Therefore, all
GUI applications launched from a terminal will become focused - even
if they don’t show any windows. Thereby the terminal loses focus.

Since is it very annoying to re-focus the terminal after each proxmark
client launch, this change makes the client unfocusable during launch
and restores the regular behaviour when a window is created.