Prevent workspace change during global fullscreen

While in global fullscreen, the workspace could be changed leaving the
fullscreen container still visible on top but losing its focus.
Extend the fullscreen command

Rather than just toggling the fullscreen modes, allow to set them
directly with:

    fullscreen enable|toggle [global]
    fullscreen disable

For compatibility, retain the previous command and its toggling behavior:

    fullscreen [global]

fixes #1120
Merge remote-tracking branch 'i3-original/next' into gaps-next