macos: allow windows to enter fullscreen
This fixes GTK's NSWindow for toplevels so that they are allowed to enter fullscreen. We were already handlign the state transitions from the setStyleMask: halper, but we didn't previously tell the window we are allowed to transition into that.
There is a bit of a mismatch here in that GTK doesn't have any such flag that determines if a window is "allowed" by policy to enter fullscreen since window managers on Linux are free to do that at will.