Asks the window manager to set the window to fullscreen, taking
all the available space. Note that other windows on the screen
may still be above as long as the window is not activated.
To be sure the window is really fullscreen, activate it.
It may be that there are some settings preventing this, so don't depend
on that it is really fullscreen after calling this function.
Normally it is, but don't write code that crashes if not.