Fix newgui double keydown event
|Reported by:||(none)||Owned by:||fingolfin|
This is a fix for newgui, which would from time to time
(randomly) recieve a double keydown event.
That keydown event was actually a keyup event (!), but
because of sdl.cpp OSystem::poll_event not filling the
event struct for the keyup event (!?) this was
Patch also reintroduces EVENT_KEYUP, and enables
handling of said event in newgui.cpp.
Ticket imported from: #582286. Ticket imported from: patches/85.