GlobalShortcut_win: use boost::future/boost::promise instead of std:: variants for better MinGW compatibility.
GlobalShortcut_win: use boost::future/boost::promise instead of std:: variants for better MinGW compatibility.

Fixes mumble-voip/mumble#3239

Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble